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Crystallization behavior of poly(vinylidene fluoride) composites containing zinc phenylphosphonate.

INTRODUCTION

In the recent years, polymer additives have often been used to improve the physical and chemical properties of polymers. Among them, nucleating agents have attracted much attention (1), (2). The crystallization process is important for the industrial use of crystalline polymers, because it significantly affects productivity and physical properties. To improve the properties of crystalline polymers, various nucleating agents have been developed recently. For polypropylene, trans-quinacridone dye is known to act as a [beta]-phase nucleating agent (3). Furthermore, sorbitol derivatives have been used to generate small spherulites and improve transparency (4). For polylactic acid (PLA), addition of zinc phenylphosphonate (ZPP) has been reported to improve crystallization temperature and rate (5). It has been reported that a PLA/montmorillonite (MMT) nanocomposite has a higher crystallization rate and heat-distortion temperature as well as better mechanical properties compared to pure PLA (6).

Poly(vinylidene fluoride) (PVDF) is a crystalline polymer that has various attractive properties including ferro-electricity, piezoelectricity, and pyroelectricity (7). The polymer shows polymorphism and has at least four crystal phases: [alpha], [beta], [gamma], and [delta] (8), (9). Among them, the [alpha]-form crystal is generally obtained by slow cooling from the melt. It has been reported that the other crystalline phases are formed through crystallization from solutions in various solvents (DMF, acetone, and HMPA) (10).

Similar to work with other polymers, the addition of inorganic or organic compounds to PVDF has also been investigated. It has been reported that benzenetrisamides promote the crystallization of PVDF (11). The crystallization behavior of a PVDF/MMT nanocomposite was investigated by Yu et al. (12). In particular, they analyzed the nonisothermal crystallization behavior of PVDF/MMT composites using the Avrami equation (13) based on Jeziorny method (14). Their analysis yielded a successful description of the crystallization behavior of PVDF and also explained the slightly complicated results for the PVDF/MMT crystallization process.

Composites such as PVDF/MMT and PVDF/clay have been investigated by many researchers. However, to the best of our knowledge, PVDF composites containing ZPP have not been investigated yet.

In this study, the crystallization behavior of PVDF composites containing ZPP (known as a nucleating agent for PLA) was examined. The synthesis and structure of ZPP, which is a metal phosphate compound, have been studied (15), (16). The crystal structure and thermal stability of ZPP were reported by Bataille et al. (17). ZPP has a layered crystal structure similar to MMT, and the intercalation reaction of ammonia or alkylamine into ZPP has been reported (18), (19). Although ZPP forms a mono-hydrate crystal (Zn([O.sub.3][PC.sub.6][H.sub.5])*[H.sub,2]O) in moist air, ZPP anhydrate (Zn([O.sub.3][PC.sub.6][H.sub.5])) was used in the experiment. ZPP monohydrate changes to the anhydrous form at 75[degrees]C, and ZPP anhydrate is stable up to 330[degrees]C.

The crystallization process of the PVDF composites containing ZPP was analyzed using polarized optical microscopy (YOM) observations and differential scanning calorimetry (DSC) measurements. A nonisothermal crystallization experiment was performed for kinetic analysis using the Jeziorny method. In addition, wide-angle X-ray diffraction (WAXD) was used to examine the composite crystal structure.

EXPERIMENTAL

The materials used in this study were commercially available. The original PVDF sample was provided by Kureha Corp. (product name: KF-1000). ZPP in white powder form was purchased from Nissan Chemical Industries (product name: Ecopromote).

Before mixing, PVDF and ZPP were dried in an oven at 80[degrees]C for 8 hr. The composites were prepared by mixing 0.25, 0.5, and 1 wt% ZPP with PVDF in a mechanical kneader (Toyoseiki Laboplastmill) at 200[degrees]C for 10 min. The composites containing 0.25, 0.5, and 1 wt% ZPP were denoted as PVDF-0.25, PVDF-0.5, and PVDF-1.0, respectively. Pure PVDF was prepared under the same conditions as a reference sample, which was denoted as PVDF-0.0.

POM observations were performed with a laser microscope (Keyence VHX-1000) and a microscope hot stage (Linkam LK300B). Samples for the POM observations were prepared by pressing a small amount of the PVDF composite between two cover glasses with a 20-pm spacer layer. The samples were heated at 200[degrees]C for 5 min to erase the thermal history and cooled from 200 to 165[degrees]C at a rate of 90t/min. Then, nucleation and crystal growth were observed isothermally.

Thermal analysis was conducted under nitrogen atmosphere using an SIT differential scanning calorimeter (DSC-7020), in which temperature was calibrated on the basis of the melting points of indium and zinc. The sample weights were 5-7 mg. The samples were heated from 20 to 220[degrees]C at 100[degrees]C/min, held at the temperature for 5 min, and then cooled at 5, 10, 15, or 20[degrees]C/min until crystallization was complete.

PVDF-0.0 and PVDF-1.0 were pressed into 0.4-mm thick sheets at 220[degrees]C and then cooled to 25[degrees]C at 20[degrees]C/min. These samples were analyzed by WAXD with Nifiltered Cu Ka radiation generated by a Bruker AXS M18XHF2.

RESULTS AND DISCUSSION

POM Observations

Figure 1 shows the polarized optical micrographs of the four samples [(a) PVDF-0.0, (b) PVDF-0.25, (c) PVDF-0.5. and (d) PVDF-1.0] cooled from 200 to 165[degrees]C at a rate of 90[degrees]C/min. In Fig. 1, optical anisotropy and PVDF crystals are not observed. The gray particles ~3 [micro]m in diameter in Fig. 1b-d are ZPP powder dispersed in PVDF. ZPP showed good dispersibility in PVDF; moreover, owing to the white color of the ZPP powder, the PVDF/ZPP composites were opaque.

Figure 2 shows the four samples crystallized at 165[degrees]C for 2 min. PVDF crystals started to generate as shown in Fig. 2.

The polarized optical micrographs of the four samples crystallized at 165[degrees]C for 5 min were presented in Fig. 3. Figure 3a shows PVDF spherulites ~30 [micro]m in diameter. It seemed that the crystallization of PVDF was almost complete in Fig. 3b-d, whereas there was room for crystal growth in Fig. 3a. As the amount of ZPP increased, the number of crystals tended to increase and hence the size of the crystals decreased. The results indicate that ZPP acted as a heterogeneous nucleating agent and promoted the generation of crystal nuclei. Each crystal in Fig. 3a has a spherical form, where the Maltese cross is clearly observed. In contrast, the form of each crystal in Fig. 3d is difficult to identify, and the Maltese cross is not apparent.

DSC Measurements

Figure 4 shows the DSC curves of PVDF-0.0, PVDF-0.25, PVDF-0.5, and PVDF-1.0 for the cooling process from the melt at 5[degrees]C/min. The exothermal peaks in the curves resulted from the crystallization of PVDF. As the amount of ZPP increased, the crystallization temperature shifted to the higher temperature side for each cooling rate. This indicates that ZPP particles acted as the nuclei of the polymer crystal and promoted the generation of PVDF crystals.

The crystallization temperatures (To) of the four samples at various cooling rates are summarized in Table 1, where [PHI] is the cooling rate and [T.sub.e] denotes the peak temperature of the crystallization curve. As the cooling rate increases, the crystallization peaks shift to the lower temperature side. This is considered to be because the rate of temperature change is faster than the motion of the polymer segments or nucleation rate.

TABLE 1. Peak temperature of crystallization ([T.sub.c]) of four
samples at various cooling rates ([PHI]).

Sample      [PHI] ([degrees]C/min)   [T.sub.c]([degrees]c)

PVDF-0.0                         5                   145.9

                                10                   143.1

                                15                   141.6

                                20                   140.3

PVDF-0.25                        5                   142.5

                                10                   149.9

                                15                   148.1

                                20                   146.5

PVDF-0.5                         5                   154.6

                                10                   151.9

                                15                   149.8

                                20                   148.3

PVDF-1.0                         5                   155.9

                                10                   153.0

                                15                   151.0

                                20                   149.2


The nonisothermal crystallization behaviors of PVDF-0.0 and PVDF-1.0 were compared to clarify the influence of ZPP addition. The various parameters of PVDF-0.0 and PVDF-1.0 obtained from the DSC curves are summarized in Table 2, where [DELTA][H.sub.c], [T.sub.cint], and [T.sub.cend] are the total crystallization enthalpy, the initial temperature of crystallization, and the end temperature of crystallization, respectively.

TABLE 2. Parameters of PVDF-0.0 and PVDF-1.0 obtained from DSC
curves.

Sample          [PHI]        [DELTA][H.sub.c]  [T.sub.cint]
          ([degrees]C/min)  (J/g)              ([degrees]C)

PVDF-0.0                 5               50.2         153.6

                        10               51.1         150.8

                        15               51.3         148.2

                        20               52.4         147.1

PVDF-1.0                 5               44.4         160.0

                        10               45.6         157.5

                        15               46.5         156.4

                        20               45.7         155.3

Sample     [T.sub.cent]    [DELTA]T
          ([degrees]C)   ([degrees]C)

PVDF-0.0          140.8          12.8

                  135.5          15.3

                  132.3          15.9

                  127.3          19.8

PVDF-1.0          149.6          10.4

                  146.7          10.8

                  143.3          13.1

                  141.3          14.0


The range of crystallization temperature ([DELTA]T) was defined as follows:

[DELTA]T = [T.sub.cint] - [T.sub.cend] (1)

The crystallization enthalpy was almost independent of the cooling rate for each sample and the crystallization enthalpy of PVDF-1.0 was slightly smaller than that of PVDF-0.0. With the addition of ZPP, [DELTA]T decreased owing to the acceleration of crystallization.

The transitional degree of sample crystallinity R(T)] during the cooling process is defined by the following equation:

X (T) = [DELTA][H.sub.T]/[DELTA][H.sub.c] (2)

Here, [DELTA][H.sub.T] is partial crystallization enthalpy at a given temperature T ([T.sub.cend] [less than or equal to] T [less than or equal to] [T.sub.cint]), which is calculated from the area of the DSC curves between T and [T.sub.cint]. Figure 5 shows the temperature dependence of X(T) for PVDF-0.0 and PVDF-1.0 during the cooling process from the melt at 5[degrees]C/min.

For the nonisothermal crystallization, the crystallization time t from the crystallization starting point could be described by the following equation:

t = ([T.sub.cint - T)/[PHI] (3)

Using this equation, the horizontal axis of Fig. 5 could be transformed from temperature to crystallization time: that is, X(T) could be described as a function of t: X(t). Figure 6 shows the relationship between the crystallization time t and degree of crystallinity X(t) during the cooling process from the melt at 5[degrees]C/min. Because of the nucleation effect, the sigmoid curve in Fig. 6 became steeper by the addition of ZPP.

A number of models based on the Avrami equation have been proposed for nonisothermal crystallization analysis (20-22). Yu et al. (12) investigated the nonisothermal crystallization of PVDF and PVDF/MMT nanocomposites and successfully described PVDF crystallization behavior by the Jeziorny method. This method was also used in this study for the nonisothermal crystallization analysis of PVDF and PVDF/ZPP.

The Avrami equation adopted by Jeziorny for the nonisothermal crystallization of polymers is described as follows:

1 - X(t) = exp(-[Z.sub.t][t.sup.n]) (4)

where n is the Avrami exponent depending on the type of crystallization and Z, is the kinetic parameter of crystallization. Equation 4 could be decomposed as follows:

log[-ln(1 - X(t))] = log[Z.sub.t] + nlogt (5)

where n can be calculated by plotting log[-ln(1 - X(t))] versus log rand evaluating the gradient of the graph. For an adaptable case, log [Z.sub.t] corresponds to the intercept of the plot with the vertical axis. For nonisothermal crystallization, the final form of the kinetic parameter [Z.sub.c] is given as follows:

log[Z.sub.c] = log[Z.sub.t]/[PHI] (6)

where [PHI] is the cooling rate.

Figure 7 shows the plots of log[-ln(1 - X(t))] versus log t for PVDF-0.0 and PVDF-1.0, where [PHI] = 5. The graph of PVDF-0.0 is almost linear, whereas a break point appeared at around X(t) = 0.6 for PVDF-1.0; that is, the slope of the line was steep in the early stage (stage I) and became a gentle in the latter stage (stage IT). Similar results were obtained at other cooling rates. For PVDF-0.0, the [Z.sub.t] and n values could be obtained from the intercepts of the vertical axis at log t = 0 and the slope of the lines in Fig. 7. For PVDF-1.0, the n values were separately calculated for the slope in each stage. The parameters obtained from Fig. 7 and the graphs of other cooling rates are summarized in Table 3. The Avrami exponent n was close to 4 for PVDF-0.0, which indicated three-dimensional growth of homogeneous nucleation. The values of n and 4 increased by adding ZPP, which acts as a nucleating agent and may affect the crystallization behavior of PVDF, as shown in Fig. 7, where the Avrami exponents n are larger than 4 in stage 1 and those in stage II (n') are 2.2-2.6. The obtained values of n and n' were not necessarily integrals, which imply that some complicated crystallization process may occur.

TABLE 3. Parameters of PVDF-0.0 and PVDF-1.0 obtained from modified
Avrami analysis.

Sample     [PHI] ([degrees]C/min)   [Z.sub.c]    n       n'

PVDF-0.0                        5        0.68   4.20

                               10        1.05   4.23

                               15        1.19   4.09

                               20        1.17   3.95

PVDF-4.0                        5        0.91   5.92   2.19

                               10        1.39   4.80   2.59

                               15        1.33   4.79   2.50

                               20        1.28   4.55   2.64


WAXD Analysis

WAXD data of ZPP powder, PVDF-0.0, and PVDF-1.0 were collected at 25[degrees]C. PVDF-0.0 and PVDF-1.0 were cooled from the melt at 20[degrees]C/min before the WAXD analysis. A WAXD photograph of PVDF-1.0 is shown in Fig. 8, and profiles of ZPP, PVDF-0.0, and PVDF-1.0 are shown in Fig. 9. The crystal structure of ZPP is monoclinic, and the lattice constants are a = 14.49, b = 5.18, c = 5.29, and [beta] = 94.81[degrees] (17). In Fig. 9, ZPP shows two diffraction peaks at 2[theta] = 12.4[degrees] (d = 7.13 A) and 18.3[degrees] (d = 4.85 A) corresponding to the (200) and (101) planes of the ZPP crystal. The values of d-spacing were calculated from the measured values of 2[theta].

The crystal structure of [alpha]-PVDF is monoclinic, and the lattice constants are a = 4.96, I.) = 9.64, and c = 4.62 A [231. In Fig. 9, PVDF-0.0 shows the typical WAXD pat-tern of an a-phase crystal, where four diffraction peaks appear at 20 = 18.0[degrees] (d = 4.93 A), 18.5' (d = 4.80 A), 20.4[degrees] (d = 4.35 A), and 27.0[degrees] (d = 3.30 A) corresponding to the 100, 020, 110, and 111 reflections of the a-PVDF crystal, respectively. The WAXD profile of PVDF-1.0 is similar to that of PVDF-0.0. However, the 100 and 020 peaks of PVDF-I.0 merged into only one peak at 18.1[degrees] (d = 4.90 A); the diffraction peaks of PVDF-1.0 were observed at 2[theta] = 18.1[degrees], 20.2[degrees], and 27.0[degrees]corresponding to the 100/020, 110, and 111 reflections of the crystal. The d-spacings of the (100) and (020) planes of [alpha]-PVDF crystal changed from 4.93 to 4.90 A and from 4.80 to 4.90 A by the addition of ZPP, when the (100) and (020) planes of PVDF crystals may be superimposed or constrained on the (101) plane of ZPP crystals. This result may indicate evidence that the epitaxial crystallization occurred, and the crystal structure of PVDF was influenced by the atomic arrangement on the (010) surface of the ZPP crystal. This epitaxial effect stabilizes the nuclei of the PVDF crystal and increases the crystallization rate, crystallization temperature, and number of crystals. Furthermore, ZPP consists of small particles (~3 [micro]m) that effectively disperse in PVDF, as observed by POM. The high dispersibility and large surface area of ZPP improve its nucleation effect.

CONCLUSION

The crystallization behavior of PVDF/ZPP composites was investigated. POM observations indicated that the addition of ZPP promoted the crystallization of PVDF. Numerous crystals, which did not show the Maltese cross, were formed by the addition of ZPP. DSC measurements also indicated that the addition of ZPP increased both crystallization rate and temperature. As a heterogeneous nucleating agent, ZPP seemed to promote the nucleation of PVDF crystals. Nonisothennal kinetics analysis was used to evaluate the effect of the nucleation agent. The Avrami exponent obtained for the PVDF/ZPP composites was greater than that of pure PVDF. This increased value of the Avrami exponent implied that a complicated crystallization mechanism may exist in the composites. Moreover, the WAXD results demonstrated that the crystal structure of PVDF was influenced by the addition of ZPP. This suggests that an epitaxial effect may exist between the crystal surfaces of the polymer and the nucleating agent.

Correspondener to: T. Tsuboi; e-mail: tsuboi.tomoya@kao.co.jp

DOI 10.1002/pen.23331

Published online in Wiley Online Library (wileyonlinelibrary.com).

[c] 2012 Society of Plastics Engineers

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Tomoya Tsuboi, Hiromu Katayama, Takashi Itoh

Department of Macromolecular Science and Engineering, Graduate School of Science and Technology, Kyoto Institute of Technology, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan
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Author:Tsuboi, Tomoya; Katayama, Hiromu; Itoh, Takashi
Publication:Polymer Engineering and Science
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Date:Apr 1, 2013
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