Polyethylene–CaCO3 hybrid via CaCO3-controlled crystallization in emulsion

Authors

  • Stanislaw Penczek,

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    1. Center of Molecular and Macromolecular Studies of Polish Academy of Sciences, 90-363 Lodz, Sienkiewicza 112, Poland
    • Center of Molecular and Macromolecular Studies of Polish Academy of Sciences, 90-363 Lodz, Sienkiewicza 112, Poland
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  • Krzysztof Kaluzynski,

    1. Center of Molecular and Macromolecular Studies of Polish Academy of Sciences, 90-363 Lodz, Sienkiewicza 112, Poland
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  • Julia Pretula

    1. Center of Molecular and Macromolecular Studies of Polish Academy of Sciences, 90-363 Lodz, Sienkiewicza 112, Poland
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Abstract

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The polymer–CaCO3 hybrid was prepared from a polymer insoluble in water.

INTRODUCTION

Mineralization, that is, the process of formation of materials from macromolecules and inorganic solids is closely related to its older-bio brother. Biomineralization is in turn the biological process by which living organisms synthesize the complex inorganic material for their hard tissue.1 One of the directions of research in mineralization is an attempt to prepare novel materials, mostly polymer–inorganic hybrids. Polymers used till now in these processes have been water soluble. In the presented Note, functionalized although insoluble in water, polyethylene was used and it is shown that it is attached to modified crystals of CaCO3.

Polymer–inorganic hybrids could be materials by themselves, like novel nanocomposites (meso?) or additives to polymers in the form of fillers and other modifiers. Preparation of these hybrids is mostly based on controlled crystallization of inorganic salts, like the frequently studied CaCO3 and Ca3(PO4)2, crystallized in water solutions from the soluble salts, bearing Ca2+ cations and the corresponding soluble salts with required anions. A typical example is formation of hybrids insoluble in water, when streams of CaCl2 and NaHCO3 are introduced simultaneously to the water solution of polymer (there are also other methods). In the following steps of crystallization polymer is becoming a part of the crystalline particles and the mechanism of this process, in which it is assumed that the first amorphous CaCO3 particles are formed, has been tentatively proposed by Coelfen et al.2 and other authors (e.g., Chujo and coworkers,3 Wegner and coworkers4).

We developed in the past several methods of the synthesis of homo- and block copolymers with chains bearing mono- or diesters of phosphoric acid. Derivatives of phosphoric acid are known to be among the most efficient crystallization modifiers of Ca salts. In the case of di- or tri-block copolymers the other blocks are also hydrophilic, usually of the poly(ethylene oxide) structure. These polymers have been used in preparation of hybrid materials, and structure of some of these materials have recently been described.5, 6 Thus, we have shown by comparing the 13C{1H} and 31P{1H} NMR spectra of the water suspension of the hybrids and the corresponding solid state spectra, that ionic parts of the block copolymers are fully imbedded in particles and nonionic blocks are “sticking” free outside.7, 8

As it has been shortly mentioned above, usually the controlled crystallization and formation of hybrids is conducted in water solutions. However, preparation of water-insoluble polymers with inorganic salts is even more interesting, as it may provide novel materials based on the larger variety of polymers.

To the best of our knowledge, the controlled crystallization involving emulsion of the water-insoluble polymer as crystallization modifier and leading to the polymer–inorganic hybrid has not yet been attempted.

EXPERIMENTAL

Materials

Polyethylene monoalcohol (PE-OH) Mn 460 (Aldrich) was used as received. POCl3 and hexane (Aldrich) were used without purification. CaCl2 and NaHCO3 (CHEMPUR) were used as received.

Phosphorylation of the PE-OH

Phosphorylation of PE-OH was performed with POCl3. The mixture of 6.0 g of PE-OH (∼0.01 mole of [BOND]OH groups) with 50 mL of POCl3 (0.54 mol) was heated under condenser at ∼100 °C for 5 h. Then, the obtained solution was cooled down to 25 °C, and the white solid was precipitated. It was separated on the Schott funnel and next put into 500 mL of water to convert the PE-chlorophosphate into PE-phosphate. The final product was isolated, washed with distilled H2O until the negative reaction for Cl, and dried in vacuum. The degree of phosphorylation was determined from the elemental analysis.

Suspension of Phosphorylated PE (PE-Ph)

The mixture of 1.0 g of PE-Ph in 1000 mL of water was refluxed with stirring (rotation speed: 200 min−1) for 48 h. During that time, suspension was formed. Suspension was stable at room temperature for 6 months (later observation was not continued).

Crystallization of CaCO3 in the Presence of Phosphorylated PE (PE-Ph)

To the vessel with boiling suspension of PE-Ph, water solutions of 0.025 M of Na2CO3 and 0.025 M of CaCl2 (adjusted to pH ∼10) were injected via two Teflon capillaries. Addition was complete when the milky emulsion disappeared, replaced by a clear solution with suspended particles. The formed hybrid particles were separated from the reaction mixture by sedimentation, washed with water, and dried in vacuum. Next, dry particles were washed several times with hot hexane and dried again.

Measurements

The 13C{1H} and 31P{1H} NMR spectra were recorded on a Bruker AC-200 spectrometer. The 13C{1H} NMR spectra for quantitative calculation were recorded with inverted gate proton decoupling mode and time delay between pulses equal to 30 s. The 31P{1H} NMR spectra were recorded with time delay D1 = 15 s. The ss 13C and ss 31P NMR spectra were recorded on Bruker DSX-300 spectrometer operating at 75.48 and 121.50 MHz, respectively. All spectra were recorded at 25 °C.

Microscopy images of the gold-coated crystalline particles of hybrids were recorded with a digital camera connected to JEOL 5500 LV scanning electron microscope (SEM).

Photon correlation spectroscopy (PCS) measurements were taken using a Zetasizer apparatus 3000HS (Malvern Instruments). The apparatus was equipped with a He–Ne laser emitting light at 632.8 nm and detector recording intensity of light scattered at 90°.

Thermogravimetric analysis (TGA) was carried out with a Hi-Res TGA 2595 Thermogravimetric Analyzer (TA Instrument) at the heat up rate of 10 °C/min, under nitrogen atmosphere in the range from 25 to 600 °C.

RESULTS AND DISCUSSION

Creation of the polymer–inorganic hybrid requires macromolecules fitted with functional group(s) able to strongly interact with inorganic component. Therefore, in this work aiming at attaching PE to CaCO3 it was first necessary to functionalize PE. The monoesters of phosphoric acid (as well as bisphosphonates) are known to interact strongly with Ca2+.

Thus, in the first step we prepared and characterized PE-OH phosphorylated at one end (PE-Ph). In the 13C{1H} NMR (invgate method) of starting PE-OH, in (Cl2CH)2 solution in the presence of the external C6D6 solvent, integrations of [BOND]CH2OH (∼64 ppm, δ) and [BOND]CH3 (∼15 ppm, δ) indicate that the content of PE-OH is approx. equal to 77%. This value is in agreement with information given by supplier (80%). Phosphorylation was performed in the straightforward way by heating PE-OH in POCl3, taking a large excess of POCl3 (POCl3/PE-OH ratio more than 50). At these conditions PE-OH is soluble in POCl3. In the ss 31P NMR spectrum (Fig. 1) of the product there is one large, sharp peak at −2.4 ppm, δ and a relatively wide peak looking as a multiplet at −15.6 ppm, δ. The ratio of integrations is equal to 1.00/0.15. The chemical shift of the large peak is typical for the monoesters of H3PO4,9 although in ss spectra there could be a difference in comparison with chemical shifts measured in solution. The smaller peak at −15.6 ppm, δ is located in the region typical for the monoesters of pyrophosphoric acid.10

Figure 1.

The ss 31P NMR spectrum of phosphorylated polyethylene (PE-Ph).

Thus, the product, according to this spectrum, is composed of monoesters of H3PO4 (1) and H4P2O7 (2).

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Hydrolysis of 2 to 1 have not been attempted, as for our further purposes (attachment to CaCO3) 2 could be at least as efficient as 1.

Elemental analysis of the product gave 3.96% of P. This could mean, that only ∼80% of phosphorylation was achieved. However, there is only 80% of PE-OH in the sample. Thus, this “∼80%” actually means full phosphorylation of the PE-OH.

Preparation and Properties of the PE-Ph Emulsion Particles

Mineralization of CaCO3 (and other salts) was studied till now with polymers soluble in H2O and proceeds consequently in water solution. In one of the practical methods, adopted in our works, two streams of water solutions of CaCl2 and Na2CO3 are simultaneously introduced from two capillaries into water solution of the polymer. In this work, these streams have not been introduced into H2O solution (in which PE-Ph is insoluble), but into the suspension of PE-Ph. This suspension was stable for months at room temperature, as described in Experimental section. The size of particles formed was determined by the PCS method (Fig. 2), and the average size of particles was found to be equal to about 400 nm with dispersity Dv/Dn = 1.41.

Figure 2.

Volume size distribution of particles of water emulsion of PE-Ph measured by PCS method.

Formation and Properties of the PE-Ph–CaCO3 Hybrids

To the best of our knowledge, these are the first hybrids of CaCO3 prepared with water-insoluble polymer. This method looks to be general and other water-insoluble polymers may form hybrids when methods of formation of their stable emulsions are found. For any practical use, concentrations of emulsions should be much higher (not attempted in this work) than described. Perhaps, phosphorylation either at the end groups or (partial) at the repeating units is one of the possible solutions.

In this work, to the hot (∼100 °C) emulsion of PE-Ph in water two streams of CaCl2 (0.025 M in H2O) and Na2CO3 (0.025 M in H2O) were simultaneously introduced (exact conditions are given in the Experimental section). Precipitate starts to be seen within a milky emulsion in a few seconds. Addition is complete when transparent upper layer replaces the emulsion, and white precipitate settles down. This was dried and then washed several times with hot hexane to remove the unreacted PE, both not functionalized PE and (if any) PE-Ph not attached to CaCO3. In the ss 13C NMR spectra (Fig. 3) of the hybrid, the chemical shift of signal is similar to the 13C NMR signal of PE-Ph alone (not shown).

Figure 3.

The ss 13C NMR spectrum of the PE-Ph–CaCO3 hybrid.

Hexane washings were collected and after the solvent evaporation the residual solid was studied by 1H and 31P NMR. According to the spectra it was PE not containing phosphorus (spectra not given). In the ss 31P of the hybrid (Fig. 4), the major peak (about −1.2 ppm, δ) is split into two close signals (−0.68 and −1.66 ppm, δ), accompanied by two pairs of side bands (each also split into two bands).

Figure 4.

The ss 31P NMR spectrum of PE-Ph–CaCO3 hybrid.

We can only assume, that “PE-Ph–CaCO3” exists in one of two structures, for example, PE-Ph–Ca and (PE-Ph)2–Ca. Thus, for example,

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These assumptions are only speculations. However, when more PE-Ph is added to this sample of hybrid, then, the signal, which we attributed to (PE-Ph)math imageCa2+ increased. This result could indicate that when excess of PE-Ph is present, the double salt is formed.

Scanning Electron Microscopy and TGA of Hybrids

The scanning electron microscopy (SEM) pictures (Fig. 5) show that this novel process of mineralization—controlled crystallization— for hybrid (polymer–inorganic formation) in emulsion indeed works and provides hybrids with imbedded polymer. In the particular system studied in this work, particles of hybrids are needles-like and, according to SEM (Fig. 5), have an average diameter equal to 200 nm and the aspect ratio equal to ∼10. As it is well known, CaCO3 crystallized from water solution without additives gives large, hexagonal crystals.

Figure 5.

SEM images of CaCO3 crystals obtained in the presence of PE-Ph. Large picture: magnification 1000× and small picture: 6500×.

TGA of the PE-Ph (Fig. 6) is shown together with TGA of PE-Ph–CaCO3 hybrid (Fig. 7). From the latter, according to the NMR spectra, the not bonded PE-Ph was removed by extraction with hot hexane. Comparison of the TGA results indicate that the peak of decomposition at 309 °C present in the PE-Ph sample is no more present in the hybrid. This means, that at 309 °C decomposition of not functionalized PE starts. The major peak at 358 °C in PE-Ph is also present in the hybrid (357 °C). Thus, this peak is related to decomposition of PE-Ph. Besides, there appears a new peak at 429 °C, which has corresponding small shoulder at the PE-Ph sample. This peak might have various origins. Perhaps, decomposition of PE-Ph at high temperature is providing a certain new product with higher heat stability.

Figure 6.

TGA plot of the PE-Ph (containing PE).

Figure 7.

TGA plot of the PE-Ph-CaCO3 hybrid.

CONCLUSIONS

For the first time, polymer–CaCO3 hybrid was prepared from a polymer insoluble in water. Thus, PE-OH was phosphorylated and as monoester of phosphoric acid was first emulsified without any additional surfactant added and used as a modifier-component during CaCO3 crystallization. Practically all of the PE-OP(O)(OH)2 was attached, giving the PE–CaCO3 hybrid, containing 10 wt % of PE.