STUDY ON STRUCTURE AND CRYSTALLIZATION OF POLYPROPYLENE GRAFTED MALEIC ANHYDRIDE/NANOCLAY COMPOSITE

Nanocomposite from polypropylene grafted maleic anhydride (PP-g-MA) and nanoclay were prepared by melt proccessing in Brabender extruder. XRD analysis shows the formation of nanostructure (intercalated and exfoliated) of nanocomposite with interlayer space (d-space) increasing from 26.3Å to 30.9Å. These structures, however, do not affect the microcrystalline structure of polymer part of nanocomposite in whole. The study on nonisothermal crystallization shows the development in 3-dimentional direction of crystallites, that means spherulite form of crystalline structure.


INTRODUCTION
Polymer nanoclay composites (PNC) are a new class of materials that attracts great attention due to their excellent mechanical, thermal properties as well as reduced gas and liquid permeability [1,2]. Among them the thermoplastic PNC (polyolefins, polyamides, polystyrene et al.) have been studied most intensively.
Polypropylene (PP) as a very popular thermoplastic resin is involved in PNC preparation due to its very good mechanical properties in combination with low price. However, pure PP hardly forms nanocomposite with nanoclay because of its nonpolarity. So, for nanocomposite preparation, PP is often modified by polar groups such as hydroxyl, maleic anhydride, etc. [3,4]. From these modified PP, the most popular is PP grafted maleic anhydride (PP-g-MA).
It is well known that PP properties depend on its crystalline structure. Subsequently, the properties of PP/nanoclay composite should also depend on PP crystalline structure. However, using PP-g-MA instead of PP inevitably lead to some changes in crystallization behavior of nanocomposite. For that reason, there are the works devoted to crystallization behavior of nanocomposites based on PP-g-MA and nanoclay and their properties as well [5 -9].
In this paper, the formation of nanostructure of PP-g-MA/nanoclay composite and its nonisothermal crystallization have been studied.

Materials
In this study, PP-g-MA is prepared in our laboratory according to [10] by melt grafting. The  Corresponding author e-mail: polymercentre@mail.hut.edu.vn 130 grafted MA content is 0.5% (by weight). Nanoclay is I28E derived from Nanocor Company (USA).
PP-g-MA/nanoclay composite was prepared by direct melt mixing in Brabender mixer (Germany) with the speed 120 rpm and temperature 165ºC. Clay loading is 6% (by weight).

Experimental methods
Structure of nanocomposite is characterised by XRD method. X-ray diffraction is performed by Bruker D5005 diffractometer operated at 40 kV and 30 mA. Scaning rate is 2º/min. from 0.6 to 40°.
Crystallisation is studied using differential scanning calorimetry (DSC) on Bahr DSC 310 calorimeter (Germany) according to ISO 11357-1: 1997 (E). The temperature is correlated with indium. For elimination of thermal history, the samples were melted at 200°C for 10 min. and then cooled at constant cooling rates of 2.5 and 10 K/min.

Formation of nanostructure
When PP-g-MA was mixed with nanoclay at high temperature and appropriate shear rate, its molecules would intercalate into silicate layers and expand the interlayer space. The formation of these structures should be expressed on XRD pattern of PP-g-MA/nanoclay composite (Fig. 1). From X-ray diffration data, one can see that the interlayer space d extend remarkably: from 26.3Å for I28E to 30.95Å for nanocomposite. That means polymer molecules were able to move into silicate layers and form intercalated structure.
Based on XRD data, some parameters of nanocomposite may be calculated, using Scherer 131 formula [11]   cos .
t -the crystallite thickeness  -X-ray wave length  -Bragg angle B -nominated peak width, defined as The average number of silicate layer (n) may be calculated roughly as: Calculated parameters are presented in Table 1. The existence of exfoliated structure may be seen also on XRD parttern (Fig. 1): charateristic peak for nanocomposite at small angle 2θ is obviously more obtuse than that of I28E.
For more detail assess to microstructure of PPNC, the comparision of XRD patterns at wide angles for PP-g-MA and PP-g-MA/nanoclay composite was made (Fig. 2).
From Fig. 2 one can see these patterns almost coincide to each other at wide angle. That mean there is no change in microstructure (crystalline) of nanocomposite. The formation of intercalated and exfoliated structures of nanoclay does not affect to microcrystalline structure on the whole.
In general, the XRD analysis of PP-g-MA/nanoclay composite shows the formation of nanostructures (exfoliated and intercalated). Beside, it also indicates the structure of polymer part outside of nanocrystallites is similar to that of virgin polymer.

Crystallization behavior of PP-g-MA/nanoclay composite
The crystallization behavior of PP-g-MA/nanoclay composite was studied by using differential scanning calorimetry (DSC). The DSC curve of PP-g-MA/nanoclay composite at cooling rate 10 K/min was presented in Fig. 3. For comparision, the DSC curve of PP-g-MA was presented, too. From exotherms on Fig. 3 obviously that PP-g-MA/nanoclay composite crytallised at temperature higher (122°C) than PP-g-MA (113°C). Besides, the exotherm of nanocomposite shows only one peak with smaller width, while PP-g-MA exotherm shows two obtuse peak. It indicates that crystallization of PP-g-MA/nanoclay composite occurs in one stage at higher rate than PP-g-MA, where crystallization has two stages. The reason of this may be the nucleation effect of nanoclay, that points out in literature [5,12,13].
For more detail study on nonisothermal crystallization of PP-g-MA/nanoclay composite, the Avrami equation is used. Although this equation is well known for isothermal crystallization, it can be successfully applied for nonisothermal crystallization of nanocomposite [5,13].
where X t -relative crystallizion degree at moment t and defined as:  (1) can be converted as: From date presented on Fig. 4 and equation (2) the crystallisation parameters were calculated ( Table 2). As clearly shown in Table 2, in both case the value of Avrami paramenter n is around 3. Due to nucleation effect of nanoclay, it may be considered that crystalline nuclei are formed immediately at the beginning of process, so crystalline structures of nanocomposite developed in 3-dimentional directions independently on cooling rate. It quite agreed with the authors [12 -14] who point out the spherulite structures of polypropylene-clay nanocomposite. The difference in values of paramater Z shows only various crystallisation degrees.

CONCLUSIONS
1. In formation of PP-g-MA/nanoclay composite, the intercalated and exfoliated structures were formed. However, in the polymer part outside of nanostructure, the crystalline character remains unchanged.
2. PP-g-MA/nanoclay composite crystallise at higher temperature with higher rate in comparison with neat PP-g-MA. Analysis of nonisothermal crystallisation show that crystalline structures of nanocomposite develop in 3-dimentional directions (in form of spherulites) independently on the crystallisation rate. It is due to nucleation effect of nanoclay in polymer matrix.