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Modification of PTFE latex with P4VP/surfactant complexes at nanoscale.

INTRODUCTION

Surface modification of polytetrafluoroethylene (PTFE) is of interest as this polymer substrate is one of the most important, amongst engineering fluoropolymers, well known for its physical and chemical inertness [1], These properties dictate use of drastic chemical and physical means for achieving the surface modification [2-5], Therefore, most of the surface modifications on PTFE are done using plasma treatment to bring in new functionalities like hydrophilicity, hydrophobicity, biocompatibility, conductivity, lubricative, and adhesive properties. This could be achieved by grafting of particular functional groups on the polymer film surface to improve the performance and make it promising in many practical applications [6-8]. For example, grafting acrylic acid (AA), 4-vinylpyridine (4VP), 1-vinylimidazole (1VI) [9] onto PTFE one can successfully add valuable functionality to PTFE and are done using plasma methods that involve high energy input [10-13] because of which there is a possibility of chain and polar group re-orientation in the surface region resulting in gradual improvement in the adhesion property of the PTFE surface. [14].

One simple method of eliminating such high energy effects that destroy the structure of PTFE is to prepare composite particles by encapsulating or blending PTFE with conventional polymers using seeded emulsion polymerization method [15].

Most importantly, the modification becomes permanent and interesting when a suitable choice of conventional polymers is made based on specific functional groups that can allow subsequent modification via coordination bonding. The intermolecular association between components in composite interfaces plays a critical role in governing the physical properties of the system. Poly-4-vinyl pyridine (P4VP) is of interest because of its neutral basic nitrogen that allows network formation and enhanced inter chain electron transfers [16]. P4VP is a perfect choice for surface modification of PTFE because reported literature has demonstrated that P4VP can be functionalized to form complexes with metal oxides and metals because of coordination bonding [17-24],

For successful metallization of PTFE surface, pretreatment is necessarily done either by modification to become hydrophilic for better wetting or roughening by excitner-laser radiation, to improve adhesion between metal particles and PTFE [25]. Arplasma pretreated PTFE was surface modified using acrylic acid and sodium salt of styrene sulphonic acid [26], Here, we have demonstrated synthesis of PTFE-P4VP/Surfactant composite latex nanoparticles wherein surface modification of PTFE is done by P4VP using in situ seeded emulsion polymerization method. As P4VP has the capability to complex with metals, such a modification may open scope for improving the adhesion strength of PTFE with metals which is important for practical applications in printed circuit boards and microelectronics packaging.

From our earlier experience [27, 28] we know that P4VP has the ability to complex with metal/metal oxide nanoparticles and the same could be expected from P4VP grafted on the surface of PTFE. This is recommended as a simple pretreatment method of PTFE surface modification without altering the original properties of PTFE. The resulting PTFE-P4VP/surfactant complex composite latex nanoparticles were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), dynamic light scattering (DLS) analysis, FTIR spectrometry, differential scanning calorimetry (DSC), and thermogravimetric analysis (TGA). A comparison of PTFE-P4VP/ surfactant complex latex nanoparticles using Potassium heptadecafluoro-l-octanesulfonate (PFS) and sodium dodecyl sulfate (SDS) was done with different WAV ratios of PTFE and 4VP monomer. Further, the possibility of metallization of the as modified PTFE was shown using Zn[Cl.sub.2] as an example.

EXPERIMENTAL SECTION

Materials

4VP was purchased from Aldrich chemicals. The 4VP was distilled under reduced pressure and stored at 5[degrees]C. Ammonium persulfate (APS), SDS, PFS, zinc chloride (Zn[Cl.sub.2]), and Triton X100 (TX100) purchased from Fluka were used without further purification. PTFE emulsion was gifted by Hindustan Fluorocarbon, Hyderabad.

Preparation of PTFE-P4VPI Surfactant Complex Composite Latex Nanoparticles

The PTFE-P4VP/Surfactant complex latex nanoparticles were synthesized by using in situ seeded emulsion polymerization with PTFE seed particles. All the polymerizations were earned out in a 500 ml five neck jacketed reactor at 75[degrees]C equipped with a condenser, a mechanical stirrer, a thermometer, and inlets for nitrogen and 4VP. Appropriate amount of PTFE latex and selected emulsifier (SDS/PFS/TX100) were introduced into the reactor containing 100 ml of deionized water at room temperature with a stirring rate of 300 rpm. The mixture was purged with nitrogen and a continuous flow of Nitrogen was maintained till the polymerization is complete. The mixture was then heated to 75[degrees]C and desired amount of 4VP was added. After additional 15 min equilibration time, APS aqueous solution (1%) was added and the mixture was reacted for 6 h. The obtained latex was purified from the unreacted monomer by repeated dialysis. The reaction was repeated to obtain PTFE-P4VP/Surfactant complex composite latex nanoparticles by varying surfactant and the W/W ratios 1:1 to 1:5 with an increment of 1 of PTFE and 4VP monomer, respectively. The detailed recipes of the reactions are given in Table 1.

Preparation of PTFE/P4VP/PFS-Metal Complexes

The preparation of PTFE/P4VP/PFS-metal complexes was carried out by treatment of PTFE/P4VP/PFS polymer powder with (5 wt%) aqueous solutions of Zn[Cl.sub.2] for 24 h at 300 K under continuous stirring.

Quantitative chemical estimation of the composition of composite particles was done by soaking 100 mg of PTFE/P4VP/ Surfactant complex latex nanoparticles in 10 ml of ethanol with stirring for 4 h. The undissolved PTFE particles were separated by centrifuging at 8000 rpm for 20 min. P4VP was separated by evaporation of ethanol under reduced pressure, and weighed.

Characterization

FT-IR spectra of composite particles were obtained by making a pellet with KBr powder using a Thermo Nicolet Nexus 670 spectrometer at room temperature. Each sample was scanned for 128 times with a resolution of 4 [cm.sup.-1] with in a spectral range of 400-4000 [cm.sup.-1]. The SEM observations were conducted on Hitachi 3000N, Japan, operated at 10 kV. The dilute latex samples were mounted on a double-sided adhesive carbon disc and sputter coated with a thin layer of gold to prevent sample from possible charging. TEM micrographs were obtained using a Philips Technai instrument, transmission microscope at an accelerating voltage of 200 kV. The samples were prepared by wetting a carbon-coated copper grid with a small drop of the dilute latex. Upon drying, it was stained with a drop of Uranyl acetate for 5 min and dried at room temperature before analysis. DSC thermograms were recorded on a DSC Q 100 Universal instrument. The samples were placed in sealed aluminum pans and initially heated at a heating rate of 10[degrees]C/ min from room temperature to 200[degrees]C in a nitrogen atmosphere; they were quenched immediately from 200 to -70[degrees]C at a cooling rate of 50[degrees]C/min to remove the previous thermal history. The samples were subsequently rescanned at a heating rate of 10[degrees]C/min from -70 to 200[degrees]C. The instrument was calibrated with indium standards before the measurements. The average sample size was 10 mg, and the nitrogen flow rate was 20 [cm.sup.3] [min.sup.-1]. Both glass-transition temperature (Tg) and melting temperature (Tm) were determined from DSC. Perkin Elmer TGA 7 instrument was used to study the thermal decomposition profiles of composite particles under nonisothermal conditions at a constant heating rate of 10[degrees]C/ min in nitrogen atmosphere from room temperature to 800[degrees]C. For TGA analysis, 8-10 mg of solid sample was taken for analysis purpose. DLS analysis was performed at 25[degrees]C, with a Malvern MAL 1004428 at a fixed scattering angle of 90, using a 10 mV He-Ne laser and PCS software for Windows (version 1.34, Malvern, UK).

RESULTS AND DISCUSSION

Preparation of PTFE-P4VPI Surfactant Complex, Composite Latex Nanoparticles

The PTFE-P4VP/surfactant composite latex nanoparticles were prepared by taking appropriate amount of PTFE latex into deionized water with selected emulsifier and addition of calculated amounts of 4VP monomer mixture at 75[degrees]C using APS as an initiator following seeded emulsion polymerization technique.

Structural Properties

The FTIR spectra of the PTFE, P4VP/ Surfactant, PTFE/ P4VP/ Surfactant complex latex nanoparticles are shown in Fig. 1A. FTIR spectrum of PTFE shows the characteristic absorption peaks of the C[F.sub.2] groups at 1155 [cm.sup.-1] (C-F asymmetric stretching mode) and 1218 [cm.sup.-1] (C-F symmetric stretching mode), and at the lower wave numbers 507 [cm.sup.-1] (C[F.sub.2] wagging mode), 548 [cm.sup.-1] (C[F.sub.2] deformation mode), and 634 [cm.sup.-1] (C[F.sub.2] rocking mode) are detected [29]. After polymerizing the 4VP monomer in the presence of the PTFE seed particles, whereas the spectrum of all the PTFE/P4VP surfactant complex latex nanoparticles was found to be different from that of PTFE. Along with the characteristic peaks of PTFE all the PTFE/P4VP/surfactant complex latex nanoparticles have shown additional bands at 1430 and 1470 cm 1 that can be assigned to skeletal vibration of (C=C) in pyridine and the characteristic pyridine ring vibrations of free uncoordinated rings was shifted to higher wave number from 1595 to 1605 cm 'because of coordination of (N.... [M.sup.+]) in pyridine [17, 18]. Similarly, a further shift of these vibrations to 1622 [cm.sup.-1] was noticed on complexation with Zn[Cl.sub.2] in Fig. IB [30]. The changes brought in by coordination of pyridine will be discussed later in thermal properties section.

Size and Morphology of the Nanocomposite Particles

SEM pictures of PTFE, PTFE/5P4VP/PFS2, and PTFE/ 4P4VP/SDS given in Fig. 2, show an increase in the size and changes in the appearance of the particles confirming the homogeneous modification of original PTFE.

TEM photographs for plain PTFE and nanocomposite particles are shown in Fig. 3. The spherical shape of PTFE particles has been retained with increased size for composites of P4VP/ SDS complexes, whereas composites of P4VP/PFS complexes changed shape to elliptical form. This could be most probably a combination of two PTFE particles held together with PFS complex. The average sizes of the PTFE particles and its composites in TEM match with DLS data. Size distribution curves in DLS (Fig. 4) were found to be narrow with the average size of 170 nm for PTFE particles which on modification with P4VP/SDS complexes increased to 270-300 nm and to 450 nm for P4VP/ PFS complexes. Fluorosurfactants have lower surface energies and lower surface tension in comparison to hydrocarbon surfactants and therefore can wet PTFE more effectively, which probably is the cause for agglomeration of particles leading to change in morphology and size. It is also known that complexation capability of ions is directly proportional to the ionic radius thus explaining the effective coordination possibility of potassium ion which has been reported up to four molecules of 4VP in comparison to the sodium ion [16, 18].

Thermal Properties

Thermogravimetric Analysis (TGA). Thermograms of unmodified PTFE and PTFE-P4VP/SDS or PFS complex nanoparticles are shown in Fig. 5. PTFE shows a clean single degradation peak at 569[degrees]C (T III), whereas the complexes show a multidegradation pattern which shows three steps for P4VP complex, surfactant, and PTFE. Unlike PTFE all the other samples show residue at the end of the thermogram indicative of blending. In case of SDS (Fig. 5A) complex TI corresponds to the degradation of the surfactant molecule and then T III for PTFE chain. However, in case of PFS (Fig. 5B) complexes, T I is that of P4VP followed by PFS surfactant T II, because fluorosurfactant is found to have greater thermal stability than P4VP and then T III for PTFE follows. It is seen that PTFE, P4VP, and surfactant molecules undergo degradation independently irrespective of the percentage of blending. However, it was marked that the percentage of PTFE weight loss was proportional to the amount of PTFE taken for complexation and is also true with PFS as well. The maximum derivative peak degradation temperatures T I, T II, and T III are listed in Table 2. It has been noticed that thermal stabilities of PTFE component in all the complexes is slightly reduced indicating modification of PTFE. The composition of PTFE and P4VP in the composite nanoparticles was estimated theoretically, experimentally, and gravimetrically (TGA) and are listed in Table 2. In Fig. 5C the degradation pattern of the [Zn.sup.2+] (5 wt%) complexed with PTFE/P4VP/PFS is shown which proves the metallization possibility as claimed. Slightly higher thermal stability was observed for the [Zn.sup.2+] complexed PTFE. In addition a thermogram of the modified PTFE in the presence of a nonionic surfactant TX100 (Fig. 5C) was also recorded for comparison. Here, only two peaks T I pertaining to the degradation of P4VP molecule followed by T II of PTFE clearly indicates the absence of the complex formation.

Differential Scanning Calorimetry (DSC). The DSC curves of unmodified PTFE and their composites are shown in Fig. 6. Glass transition data of all the as synthesized composite nanoparticles are listed in Table 3. PTFE is a semicrystalline polymer and it is known to show crystal-crystal transitions between 19[degrees]C and 30[degrees]C and a melt transition at 327[degrees]C (ref-polymer). In the present study, it has shown the crystal-crystal transition peak at 21[degrees]C and a specific endothermic peak at 337[degrees]C, which corresponds to melting or a first-order phase transition of the fluoropolymer (this peak is not shown in DSC spectrum). The intensity of the PTFE crystal transition peak is found to decrease proportional to its content in the composite. A shift in the transition temperature to lower temperatures by 3-10[degrees]C was noticed for the composites which could be because of the plasticization effect of P4VP complexes. We had recorded the glass transition of P4VP complexes with SDS (116[degrees]C) and PFS (142[degrees]C) to monitor their thermal behavior in the absence of PTFE. Nanocomposites of PTFE and P4VP surfactant complexes have shown enhancement of Tg in comparison to their corresponding P4VP complexes. Such behavior is indicative of strong interaction between the two components of the composite resulting in the hike [17]. An increase in PFS concentration of the complex had shown enhancement in Tg. In case of PTFE composites with P4VP/SDS complexes, 1:3 composition had shown the maximum Tg. These results suggest that the two components of the composite are compatible and coordination complex formation of Pyridine N with metal salts/surfactants increases the glass transition temperature of the composite nanoparticles.

CONCLUSIONS

A simple, convenient, and nondestructive method for uniform modification of PTFE latex particles at nanoscale was demonstrated by synthesizing PTFE-P4VP/SDS or PFS complex, nanocomposites using in situ seeded emulsion polymerization technique. Formation of composite particles was confirmed by FTIR, TEM, and DLS have shown the changes in structure, size, and morphology of the composite. The composites have shown a multistep degradation pattern that may be attributed to the P4VP, surfactant, and PTFE backbone degradation. Lowering of crystal--crystal transition temperature of FTFE was noticed in the composites because of incorporation of the blended material in to the amorphous regions of PTFE causing chain segmental motion. Tg of all the complexes increased because of coordination complexation of pyridine nitrogen with the metal ion of the surfactants. Thus, a simple and easy way to modification of PTFE with P4VP towards improving its adhesion strength and compatibility and also further possibility of metallization is shown that can avoid stringent pretreatment methods like plasma, ozone or irradiation techniques that are otherwise regularly used.

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Ch. Koti Reddy, D. Shailaja

Polymers & Functional Materials Division, Indian Institute of Chemical Technology, Hyderabad 500007, Andhra Pradesh, India

Correspondence to: D. Shailaja; e-mail: sdonempudi@iict.res.in

DOI 10.1002/pen.23911

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

TABLE 1. Recipe for synthesis of PTFE-P4VP/
surfactant complex composite nanoparticles.

Sample code       PTFE (g)   4VP (g)   SDS (g)   TX 100(g)

P4VP/PFS             --         6        --         --
PTFE/5P4VP/PFSl      1          5        --         --
PTFE/5P4VP/PFS2      1          5        --         --
PTFE/5P4VP/PFS3      1          5        --         --
P4VP/SDS             --         6         1         --
PTFE/P4VP/SDS        3          3         1         --
PTFE/2P4VP/SDS       2          4         1         --
PTFE/3P4VP/SDS      1.5        4.5        1         --
PTFE/4P4VP/SDS      1.2        4.8        1         --
PTFE/5P4VP/SDS       1          5         1         --
PTFE/2P4VP           2          4        --          1

Sample code       FS (g)   APS (g)   D[H.sub.2]O

P4VP/PFS            1       0.06         100
PTFE/5P4VP/PFSl    0.5      0.05         100
PTFE/5P4VP/PFS2    0.75     0.05         100
PTFE/5P4VP/PFS3     1       0.05         100
P4VP/SDS            --      0.06         100
PTFE/P4VP/SDS       --      0.03         100
PTFE/2P4VP/SDS      --      0.04         100
PTFE/3P4VP/SDS      --      0.045        100
PTFE/4P4VP/SDS      --      0.048        100
PTFE/5P4VP/SDS      --      0.05         100
PTFE/2P4VP          --      0.04         100

TABLE 2. TGA values of PTFE, P4VP/ surfactant, PTFE-P4VP/ surfactant
complex latex.

                      T I            T II          T III
Sample code       ([degrees]C)   ([degrees]C)   ([degrees]C)

PTFE                                               569.02
P4VP/PFS             315.08         413.15           --
PTFE/5P4VP/PFS1      325.23         447.49         547.61
PTFE/5P4VP/PFS2      323.12         452.76         540.24
PTFE/5P4VP/PFS3      324.23         442.22         533.91
P4VP/SDS             291.29         374.28           --
PTFE/P4VP/SDS          --           353.69         559.21
PTFE/2P4VP/SDS         --           364.27         558.15
PTFE/3P4VP/SDS       299.94         367.39         557.10
PTFE/4P4VP/SDS       297.83         369.50         557.10
PTFE/5P4VP/SDS       296.78         366.34         556.05

TABLE 3. DSC values of PTFE, P4VP/ surfactant, PTEF-P4VP/ surfactant
complex latex.

                       PTFE crystal-
                     crystal transition        P4VP Tg
Sample code       temperature ([degrees]C)   ([degrees]C)

PTFE                       21.65                  --
P4VP/PFS                     --                 142.54
PTFE/5P4VP/PFS1            12.12                145.06
PTFE/5P4VP/PFS2            12.16                148.28
PTFE/5P4VP/PFS3            14.00                158.31
P4VP/SDS                     --                 116.11
PTFE/P4VP/SDS              17.36                101.64
PTFE/2P4VP/SDS             15.46                130.12
PTFE/3P4VP/SDS             15.59                132.25
PTFE/4P4VP/SDS             15.02                130.08
PTFE/5P4VP/SDS             13.67                121.15
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Date:Mar 1, 2015
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