Elsevier

Thermochimica Acta

Volume 618, 20 October 2015, Pages 26-35
Thermochimica Acta

Simultaneous crystallization and decomposition of PVA/MMT composites during non-isothermal process

https://doi.org/10.1016/j.tca.2015.09.009Get rights and content

Highlights

  • PVA/MMT decomposition during melting–crystallization is firstly confirmed.

  • MMT can slow down the crystallization and degradation of PVA/MMT composites.

  • PVA/MMT composite crystallization is severely affected by its degradation.

  • PVA/MMT crystallization can still be corroborated well with classical models.

Abstract

Decomposition of poly(vinyl alcohol)/montmorillonite clay (PVA/MMT) composites during melting–crystallization was experimentally confirmed by morphology and molecular structure changes. In particular, FTIR spectra show the shift of Osingle bondH stretching band as well as enhanced intensities of Csingle bondO stretching and CH2 rocking vibrational modes. Furthermore, Raman deconvolution indicates that Csingle bondH wagging, CH2single bondCH wagging, CHsingle bondCO bending and CH2 wagging modes in amorphous domains were all decreased greatly. Moreover, this decomposition leads to decreased melting enthalpy, melting point, crystallization enthalpy and crystallization temperature. Crystallization analysis shows that the MMT incorporated slows down the crystallization process in the PVA matrix regardless of the nucleation capability of MMT. Despite the severe decomposition, the crystallization kinetics still corroborated well with common classical models. As a result, molecular structure changes and crystallization retardation observed in this study clearly indicate the strong effects of the thermal degradation on the non-isothermal crystallization of PVA/MMT composites.

Introduction

Polymer crystallization under different thermal conditions is of vital importance for the optimization of processing parameters in order to achieve certain specification in the finished polymer products [1]. In principle, the heat flow is produced over the course of the phase transition during crystallization, which can be monitored using differential scanning calorimetry (DSC) technique. Subsequently, the fundamental physical parameters that are of crucial importance for theoretical and applied research including crystallization rate [2], nucleation constant [3], [4], equilibrium melting point [4], [5], [6], lateral and fold surface free energy [7], can be obtained using related physical modeling and mathematical processing approaches. In most productions, polymers are usually processed under dynamic, non-isothermal conditions such as extrusion. Therefore, insights into non-isothermal crystallization kinetics would lead to a deeper understanding to underpin further development of advanced composite materials based on polymer matrix.

In the past decades, there have been a number of reports studying the crystallization kinetics of poly(vinyl alcohol) (PVA) and PVA-based composites including partially hydrolyzed PVA [8], PVA/SiO2 [9], [10], PVA/carbon nanotubes [11], [12], PVA/polyamide 6 blend [13], and PVA/attapulgite [14]. While most reports focused on crystallization kinetics, only a few of them reported the possible degradation and its effects during the PVA crystallization [8], [12], [15], [16]. Among them, Peppas and Hansen first reported that there was no evidence of degradation observed for PVA (99% hydrolysis) during an isothermal crystallization process [15]. Subsequently, a study claimed that an actual melting equilibrium was observed without any degradation of PVA using the high vacuum DSC method [16]. The report, however, led to a speculation on how one could achieve a high vacuum within the DSC cell [8]. In 2004, Probst et al. reported that there was degradation involved during the crystallization of PVA/carbon nanotubes based on the failed repeatability of crystallization exotherms at different cooling cycles [12]. Later on, Huang et al. found, based on their FTIR spectroscopic results, that there was no evidence of degradation observed for PVA (80% hydrolysis) during the DSC and suggested the necessity to carry out non-isothermal crystallization analysis [8]. Among a number of subsequent studies on the crystallization of PVA and PVA composites [15], [16], [17], [18], experimental evidences showed that the non-isothermal crystallization of PVA–graphene composites is the combination of non-isothermal crystallization and non-isothermal degradation processes [17], [18].

Layered montmorillonite (MMT) as a very widely distributed clay material is originated from the devitrification and chemical alteration of glassy volcanic ash or tuff [19]. It belongs to the group of expandable 2:1 layer silicate minerals and exhibits the composition Nax(Al2−xMgx)(OH)2Si4O10 (derived from Al2(OH)2Si4O10) where the aluminum and magnesium ions occupy an octahedral sheet, between two tetrahedral silicate sheets [20], [21]. Due to its non-toxicity, high strength, great abundance and low cost, MMT has been widely used for the fabrication of high performance polymer composites [22], [23], [24], [25]. Yet, there was no investigation of the non-isothermal crystallization of PVA/MMT composites ever reported. In this study, the non-isothermal crystallization of PVA/MMT composites and its potential decomposition was systematically investigated for the first time using multiple-cycle DSC, FTIR and Raman techniques, to gain a better understanding into the crystallization mechanism of this particular composite material.

Section snippets

Materials

PVA (Mw  145,000 and 98–99% degree of hydrolysis) was purchased from Sigma–Aldrich (Sydney, Australia). Montmorillonite clay Cloisite® Na+ (MMT) was provided by Southern Clay Products, Inc. (TX, USA). All other chemicals used in this study were of analytical grade.

Fabrication of PVA/MMT composites

Aqueous solutions of 5 wt.% PVA were prepared by dissolving PVA in deionised water for 3 h at 95 °C, which was then cast in a plastic Petri dish and dried in an oven at 40 °C for 3 days to obtain a homogenous reference film. For the

Morphology

The original surface morphology of the PVA/MMT composites (i.e. PM-0.3 and PM-1.0) and the neat PVA films were investigated by the optical microscopy (Fig. 1(a) and Fig. S1(a) and (b)). As expected, these films were shown to be homogenous despite some small holes (about 10 μm) observed, which may be originated from the bubbles during the drying process. However, a number of big holes (more than 80 μm) were also observed inside the neat PVA and the PVA/MMT composites as shown in Fig. 1(b) and (c)

Conclusions

The non-isothermal study of the neat PVA and PVA/MMT composites indicates the weight loss percentage can be as high as 5% during the first heating–cooling cycle, and keeps increasing with prolonged heating–cooling cycle. The introduction of MMT not only helps the thermal stability of PVA/MMT composites but also acts as an effective heterogeneous nucleation, promoting the crystallization of PVA/MMT composites. On the other hand, severe degradation retards the crystallization of neat PVA and

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