Brazilian Journal of Chemical Engineering EFFECT OF POLY ( N-VINYPYRROLIDONE ) ON THE NON-ISOTHERMAL CRYSTALLIZATION KINETICS AND VISCOELASTIC PROPERTIES OF PVDF FILMS

Poly(vinylidene fluoride) (PVDF) and PVDF blends with various molecular weights of poly (Nvinylpyrrolidone) (PVP) films were prepared in dimethyl formamide through the solution casting method. Non-isothermal melt crystallization studies of PVDF films were carried out by cooling the molten samples at different temperatures using differential scanning calorimetry (DSC). The obtained films have been characterized by dynamic mechanical thermal analysis (DMTA). Crystallization kinetics of PVDF films were successfully described by the Jeziorney, Mo and Ziabicki models. The Ozawa equation was found to be invalid for describing the crystallization kinetics. Kinetic parameters such as t1/2, Zc and F(T) indicated that the crystallization rate decreased for PVDF/PVP films as compared to neat PVDF films and was affected by the molecular weight of PVP. The results based on Ziabicki's model revealed that the addition of PVP decreased the ability of PVDF to crystallize under non-isothermal melt crystallization conditions. The activation energy was calculated through Friedman and advanced isoconversional methods. Results showed that the addition of PVP to PVDF films caused an increase in activation energy. By comparing DMTA results of PVDF/PVP blends with neat PVDF films, it could be concluded that blending PVDF with PVP caused an increase in the glass transition temperature (Tg) while the storage modulus was decreased.


INTRODUCTION
Polyvinylidene fluoride is popular in industrial applications as a semi-crystalline polymer due to favorable properties like good thermal stability, chemical resistance, high mechanical strength and ferro-electricity (Nasir et al., 2007;Sencadas et al., 2010).The crystalline structures of PVDF polymer, according to the chain conformation with trans or gauche linkages, are α, β and γ phases (Nasir et al., 2007).The crystalline morphology and crystallinity of PVDF are of major importance in various applications.However, the hydrophobic characteristic of PVDF is a limitation in some applications.For example, membrane fouling caused by hydrophobic interactions results in rapid water flux decline and high energy-consumption, especially when the wastewater contains natural organic matter (NOM), proteins and micro-organisms (Rajabzadeh et al., 2012).

Brazilian Journal of Chemical Engineering
Hydrophobic characteristics of polymers can be altered by polymer blending, that is more effective and widely used in comparison to chemical synthesis procedures (Ma et al., 2008;Li and Xu, 2012;Li et al. 2013).Studies showed that the crystalline phase of PVDF can be changed by blending with amorphous polymers exhibiting physical interaction with PVDF.Therefore, it is necessary to investigate the crystallization kinetics for optimizing the process conditions and improving the structure-property correlation.The main focus has been on the crystallization behavior of PVDF and its blend in melt crystallization processes in the open literature (Lee and Ha, 1998;He et al., 2008;Zhong et al., 2011).Sencadas et al. (2010) studied the isothermal melt crystallization of PVDF at different crystallization temperatures.The Avrami parameters and the Hoffman-Weeks model were discussed to obtain the equilibrium melting temperature.The crystallization and morphological behavior of PVDF/polyhydroxybutyrate blends were also studied by Liu et al. (2005).They described a phase diagram by calorimetric measurements and reported the Avrami exponent for pure PVDF, which has a value of approximately three.The miscibility behavior of poly(methyl methacrylate) and PVDF was investigated by Fan et al. (2007).They found that the Avrami exponent decreases with rising crystallization temperature.Mancarella and Martuscelli (1977) also reported that the PVDF Avrami exponent variation was between 2.99 and 4.60 and that the half crystallization time was increased by an increase in crystallization temperature.Gradys et al. (2007) studied non-isothermal crystallization of PVDF at ultra high cooling rates.Their results indicated that pure β-phase of PVDF was obtained during the melt-crystallization process at cooling rates above 2000 K/s.
Although, there are some reports in the literature on the crystallization behavior of PVDF, less attention was paid to the non-isothermal crystallization kinetics of PVDF/PVP blends.In this study, the effect on PVDF crystallization of PVP with various molecular weights as an amorphous and water soluble polymer was investigated.The PVDF/PVP blend is a miscible system due to the compatibility of the two polymers (Chen and Hong, 2002;Ji et al., 2008;Freire et al., 2012).Dynamic mechanical thermal analysis (DMTA) was used to characterize the dynamic mechanical properties of PVDF films.The Jeziorney, Ozawa, Mo and Ziabicki kinetic models were applied to describe the crystallization behavior of PVDF films.Furthermore, the activation energy of melt crystallization for sample films was determined from the Freidman equation and advanced isoconversional method.

Sample Preparation
PVDF films were prepared using the solvent casting method.Poly(vinylidene fluoride) pellets were dissolved in DMF at 50 °C for 10 h and then poly(Nvinylpyrrolidone) was added at a PVDF: PVP weight ratio of 1:1.The PVDF solution was poured into a flat dish for solvent evaporation at room temperature within an interval of two weeks.The samples were dried further at 50 °C for 8 h to remove the solvent residues.The thicknesses of polymer films were about 150-200 µm.The samples were named neat PVDF, PVDF/PVP1 for PVP with low molecular weight and PVDF/PVP2 for PVP with high molecular weight.

Characterizations
The crystallization kinetics of PVDF polymer and the effects of PVP on its crystallization behavior were evaluated using a differential scanning calorimeter (DSC) (Polylabe 625, instrument, UK).The weights of all samples were ~5 mg.The samples were heated from room temperature to 200 °C with a 30 °C min -1 heating rate and held for 3 min to eliminate the previous thermal history.Subsequently, the samples were cooled to 50 °C at predetermined rates.The non-isothermal process included melt crystallization at different cooling rates: 2.5, 5, 10 and 20 °C min -1 , while exothermal curves of heat flow were recorded as a function of temperature and the experiments carried out under nitrogen atmosphere.FTIR spectra were obtained by a FTIR instrument (Bruker, model Equinox) in the 600-3500 cm -1 wave number range.Dynamic mechanical thermal analyses of the samples were carried out using a DMA, (Tritec 2000 machin) under the bending mode at a frequency of 0.1 Hz.The temperature range was from -100 to 100 °C at a rate of 5 °C min -1 .The dried polymer films were cut into approximately 2.5×1×0.15cm 3 rectangles.The amplitude was set to be within the linear viscoelastic regime.

FTIR Spectr
For a bette PVDF and P roscopy was PVDF and its nd 890 cm -1 t 1160 cm   The relative on of the cry termined from ers by parti herms.

Ozawa Meth
The Ozaw pproaches fo roposed by e 978).This m he non-isothe ided into sm ion is express

Friedman Eq
The Fried Ma et al.,