Article Cite This: Macromolecules 2019, 52, 4270−4277
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Aqueous Dispersions of Ethylene Copolymers and Their Laponite Clay Nanocomposites from Free-Radical Dispersion Polymerization Tobias O. Morgen,† Hendrik Luttikhedde,‡ and Stefan Mecking*,† †
Department of Chemistry, University of Konstanz, Universitätsstraße 10, 78457 Konstanz, Germany BYK Netherlands BV, Danzigweg 23, 7418 EN Deventer, The Netherlands
‡
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S Supporting Information *
ABSTRACT: Aqueous dispersions of ethylene copolymers with polar comonomers are of practical interest, e.g., as additives for paints or water-based adhesives. We report insights on the free-radical heterophase copolymerization of ethylene with vinyl acetate, acrylates, methacrylates, acrylamides, and ionic vinyl or (meth-)acrylate comonomers. Copolymers with polar comonomer contents of 0.1−10 mol % were obtained via a surfactant-free dispersion copolymerization at relatively mild conditions (250 atm, 85 °C). This allows for tuning of the low-density polyethylene particle surface properties, as demonstrated for polymer nanocomposite particles with Laponite clay in a Pickering-stabilized dispersion polymerization approach. Mechanistic insights on particle formation and stabilization are discussed.
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INTRODUCTION
be improved by incorporation of polar groups into the PE particles by incorporation of suitable comonomers. In nonaqueous media, linear ethylene copolymers with most fundamental monomers (except for methyl methacrylate, MMA), vinyl and allyl compounds are accessible via catalytic routes using neutral phosphinesulfonato Pd(II)1,10−12 or phosphino-phenolate Ni(II)13,14 catalysts. To date, the few examples of catalytic copolymerizations of ethylene with polar comonomers in water are restricted to reactions with methyl acrylate15 or carbon monoxide16 to acrylate functionalized PE or strictly alternating polyketones, respectively. Functionalized, branched low-density polyethylene (LDPE) analogues are accessible via free-radical copolymerization as studied especially for vinyl acetate,17 (meth-)acrylates, and carbon monoxide18−20 as comonomers. These ethylene copolymers are produced in high-pressure reactions on an industrial scale and are further processed to packaging films. Poly(ethylene-covinyl acetate) (EVA) with less than 40 wt % vinyl acetate is applied in packaging, as a fuel additive or hot melt adhesive. Aqueous EVA dispersions are accessible via an emulsion copolymerization of ethylene and vinyl acetate.21,22 Freeradical copolymerization of ethylene with other monomers like acrylates, methacrylates, or acrylamides is challenging since their copolymerization parameters rx = kx−X·kx−ETHYLENE−1 are 8−18, whereas the ones of ethylene rethylene = kethylene−ETHYLENE· kethylene−X−1 are 0.03−0.06 (with X, ETHYLENE: monomer; x, ethylene: last incorporated repeat unit).23 Therefore, respective batch copolymerizations tend to form copolymer mixtures
Polyethylene (PE) is the largest-scale synthetic polymer material produced. Copolymers of ethylene with polar comonomers are of interest because already small concentrations of polar groups in PE have an effect on crucial surface properties.1 Tuning of the latter is crucial for systems in which particle−particle-interactions are relevant. This applies to, e.g., aqueous, surfactant-free Pickering-stabilized polymerization with silica particles which, besides physical post-polymerization mixing methods,2 is an efficient route to obtain dispersions of polymer nanocomposite particles. The obtained latexes are highly dispersed precursors for the preparation of, e.g., composite films, which combine properties of hard inorganic and soft polymeric materials. The influence of the polymer particle chemical nature on the stabilization efficiency of Pickering stabilizers has been investigated in detail for Pickering emulsion copolymerizations of styrene, acrylates, methacrylates, and vinyl acetate.3−8 Especially, particles that contain a certain degree of (co)polymer of the latter three monomers were found to possess favorable interaction with silica particles leading to sufficient adsorption on the polymer surface. This allows for a Pickering stabilization of the particles, eliminating the need for an additional organic surfactant. We recently showed that colloidally stable nanocomposite particle dispersions of low-density polyethylene (LDPE) and the synthetic smectite clay Laponite with polymer contents of >20% are accessible via a free-radical Pickering dispersion polymerization at relatively mild conditions (250 atm, 85 °C).9 However, due to the high hydrophobicity of PE, the degree of coverage of clay on the polymer particles is limited. This raises the question whether the Laponite stabilization efficiency can © 2019 American Chemical Society
Received: February 12, 2019 Revised: May 6, 2019 Published: May 28, 2019 4270
DOI: 10.1021/acs.macromol.9b00305 Macromolecules 2019, 52, 4270−4277
Article
Macromolecules
that the bulk composite materials contain the same mass ratio of polymer to clay as the respective dispersions. This allowed the calculation of the polymer content in the precipitated composites from the total mass of precipitated composite mtot and the total mass of clay added to the initial reaction mixture mclay0 with (mtot − mclay0) mtot−1. NMR spectra were recorded on a Bruker Avance III 600 spectrometer. The chemical shifts were referenced to the respective solvent signals. The high-temperature measurements of polymers were carried out in 1,1,2,2-tetrachloroethane-d2 at 100 °C (1H NMR: 6.00 ppm, 13C NMR: 73.78 ppm). Resonances were assigned according to refs28−31. Incorporations of polar comonomers were determined from their α-methylene signals in the polymer backbone relative to the total signal intensity from the polyethylene segments. Attenuated total reflection (ATR)-IR spectra of precipitated polymer samples were recorded on a Spectrum 100 by Perkin Elmer. For calculation of the comonomer incorporation, the ratio of the intensity of the CO signal [1720−1740 cm−1 for (meth-)acrylates, 1640− 1660 cm−1 for acrylamides, 1550 cm−1 for sodium (meth-)acrylate] to the intensity of the C−H signal of the PE at ∼2915 cm−1 was determined and referenced to polyketone samples with known CO content synthesized via acyclic diene metathesis copolymerization (Figure S1 in the Supporting Information).32 General Dispersion Copolymerization Procedure. Dispersion copolymerizations were carried out similar to a previously reported procedure.9 The respective amount of Laponite-RD clay was added to 150 mL of deionized water, which was stirred vigorously. The mixture was then stirred for 1 h, upon which it became completely clear. The respective amount of potassium persulfate was dissolved in 4 mL of deionized water and added dropwise to the clay dispersion. Optionally, sodium bicarbonate (0.5 g, 5.95 mmol) dissolved in 4 mL of deionized water was added to the reaction mixture. Higher amounts of added sodium bicarbonate, or potassium persulfate, resulted in a visible increase in turbidity and viscosity of the clay dispersions. Clay-free reaction mixtures were prepared by directly adding the respective initiator and, optionally, sodium bicarbonate to 150 mL of water. Oxygen was removed from all dispersions by bubbling nitrogen through the stirred mixtures for 2 h. Comonomers were added afterward, and the mixture was stirred until they were completely dissolved. Polymerization experiments were performed in a 285 mL Limbo reactor by Büchi, Uster, constructed from Hastelloy equipped with a Cyclone 075 stirrer, in- and outlet valves, a temperature sensor, and a digital pressure sensor (Figure S2). The autoclave was evacuated and purged with nitrogen three times before the clay dispersion was transferred into the reactor via cannula. The desired stirring rate was adjusted, and the temperature was set to 23 °C. The reactor was pressurized with ethylene to 110 bar. Compression of the supercritical fluid ethylene was carried out with a Teledyne Isco 260D Syringe Pump equipped with a cooling mantle. After equilibrating the system for half an hour, the reactor was closed and the internal temperature was raised to 85 °C within 30 min. Final pressures were 230−290 bar. In all experiments, these did not drop below 120 bar, which ensured a roughly comparable ethylene concentration in the water phase over time for all polymerizations. The reactions were stopped by careful venting followed by cooling to room temperature. For determination of the solids contents and for analysis of the polymer properties, 50 mL of the dispersion was precipitated by addition of 0.25 M tetra-n-butylammonium bromide (Bu4NBr) solution. The bulk polymer was filtered off, washed with water, dried under vacuum, and weighed. Since Laponite precipitates with the polymer under these conditions,9 the respective amount of Laponite-RD was subtracted to calculate the polymer content from the solid content. Contact Angle Measurements. The static contact angles of water and diiodomethane on films of clay-free copolymers were determined using a Drop Shape Analyzer DSA25 by KRÜ SS and the included software. For both liquids, the contact angles were averaged from 10 measurements between 5 and 30 s after dropping. The software automatically calculates the dispersive and the polar surface energy from the two angles obtained. The films were prepared by drop-casting of diluted copolymer dispersions (0.5 wt % polymer
with a pronounced composition drift, namely, (meth-)acrylateor acrylamide-rich polymer material initially and, as soon as the polar comonomer concentration in the reaction medium diminishes, ethylene-rich polymers at a later reaction stage. Copolymers synthesized by continuous high-pressure freeradical copolymerization of ethylene with acrylic esters or acrylic acid are commercially relevant products.24−27 We now report the surfactant-free dispersion copolymerization of ethylene as a route to obtain clay nanocomposite dispersions of ethylene copolymers or stabilizer- and clay-free copolymer dispersions. A range of polar and ionic comonomers were studied to understand their influence on the particle formation mechanism and the colloidal stability both in the presence and absence of the Pickering stabilizer Laponite.
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EXPERIMENTAL SECTION
Materials. Ethylene of 4.5 grade was supplied by Air Liquide. Acrylamide (AA; 97%), 2-hydroxyethyl methacrylate (HEMA; 97%), methyl acrylate (MA; 99%), methyl methacrylate (MMA; 99%), poly(ethylene glycol) methyl ether acrylate (PEGA; Mn 480 g mol−1), potassium persulfate (KPS, >99%), sodium acrylate (Na-A; 97%), sodium methacrylate (Na-MA; 99%), sodium polyacrylate (Mw 5100 g mol−1), sodium polymethacrylate (40 wt % aqueous solution, Mw 4000−6000 g mol−1), sodium 4-vinylbenzenesulfonate (Na-VBS; >90%), sodium vinylsulfonate (Na-VS; 25 wt % aqueous solution), and (vinylbenzyl)trimethylammonium chloride (VBTMAC; 99%) were purchased from Aldrich. Sodium bicarbonate (>99%) and vinyl acetate (VAc; >99%) were supplied by Fisher Scientific. Tetra-nbutylammonium bromide (98%) and N-isopropylacrylamide (NIPAM; 98%) were purchased from abcr. Byk Additives & Instruments provided Laponite-RD. The stabilizers were removed from the monomers HEMA, methyl acrylate, methyl methacrylate, and vinyl acetate by passing them through a column filled with silica gel. NIPAM was recrystallized from a toluene/n-hexane mixture and dried under vacuum at 40 °C. The other monomers were used as received. Analytical Methods. For dynamic light scattering (DLS), a Malvern Nano-ZS ZEN 3600 particle sizer (173° backscattering) was used. Diluted samples were measured at 25 °C. The autocorrelation function was analyzed using the Malvern dispersion technology software 7.12 algorithm, yielding number-weighted particle size distributions and the polydispersity index (PDI), which is a measure for the width of the particle size distribution. ζ-Potential measurements were performed on the same instrument. Samples were diluted to a solid content of 0.1 wt % for this purpose. Differential scanning calorimetry (DSC) was performed on a Netzsch DSC 204 F1 with a heating/cooling rate of 10 K min−1 on bulk samples. For material containing known amounts of clay, the intensities of the DSC curves were corrected with a factor of (mPE + mclay)·mPE−1. Transmission electron microscopy (TEM) images were acquired on a Zeiss Libra 120 EF-TEM instrument (120 kV). The respective samples were diluted to a solid content of 0.03 wt % and dialyzed in a Spectrum Laboratories Spectra/Por Dialysis Membrane 1, molecular weight cutoff 6000−8000 with deionized water for at least 2 days. The resulting dispersions were dropped onto a TEM copper grid and dried for 2 h. Average particle sizes from TEM images were determined by measuring the average diameter of 100 particles. Gel permeation chromatography (GPC) was measured on a precipitated bulk polymer in 1,2,4-trichlorobenzene at 160 °C at a flow rate of 1 mL min−1 on a Polymer Laboratories 220 instrument with Olexis columns. Differential refractive index, viscosity, and light scattering (at 15 and 90°) were used for detection. The measured elution volumes were evaluated with the PL GPC-220 software by calibration of the instrument with narrow distributed PE standards. Thermogravimetric analysis (TGA) was performed on an STA 429 instrument by Netzsch. The samples were heated up to 670 °C under nitrogen in a first step and then to 920 °C under oxygen to burn residual carbon compounds. TGA analysis of precipitated polymer samples reveals 4271
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Table 1. Pickering-Stabilized and Clay-Free Dispersion Copolymerizations of Ethylene and Different Types and Amounts of (Meth-)acrylate Comonomersa entry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
comon.
comon. (mmol)
Laponite (g) 1.5
MMA MMA MMA MMA MMA MMA HEMA HEMA MA MA MA MA MA MA PEGA PEGA
5 10 20 5 10 20 10 10 2.6 5 10 2.6 5 10 1.7 1.7
1.5 1.5 1.5 1.5
1.5 1.5 1.5 1.5
yield (g)
dp (nm)b
PDIb
peak Tm (°C)c
cryst. (%)c
Mw (103 g mol−1)d
Mw/Mnd
7.8 16.0 8.4 7.7 7.3 21.3 22.7 18.6 7.1 3.9f 6.3 5.8 1.4 17.4 14.2 7.1 11.2 4.9f
120 (120) 98 (94) 95 (90) 81 (85) 83 (60) 58 (58) 58 (59) 72 (66) 38 (45) 41 (−) 86 (90) 72 (74) 43 (47) 55 (55) 54 (49) 50 (46) 71 (70) 103 (97)
0.02 0.02 0.01 0.02 0.04 0.11 0.08 0.12 0.20 0.42 0.02 0.02 0.08 0.11 0.16 0.46 0.06 0.08
102 99 102 103 103 100 96 95 99 81 102 102 95 96 96 99 103 102
25 11 27 25 19 25 23 20 21 6 21 26 12 26 25 24 32 33
22.5
4.4
g
27.6 19.9 23.1
g
7.6 5.3 5.4
g
g
g
g
g
g
21.1 g
16.3 25.1 10.8
F (mol %)e
8.1 g
7.9 7.7 6.6
g
g
g
g
g
g
30.4
10.3
g
g
0.7 (1.3) 2.1 (3.0) 5.3 (5.0) −g (0.5) −g (0.6) −g (2.4) 2.3 (3.0) −g (7.6) 0.2 (0.7) 1.4 (1.8) 6.0 (5.8) −g (0.1) −g (0.8) −g (2.7) n.d. (0.3) −g (0.2)
a Reaction conditions: 85 °C reaction temperature, 2 h reaction time, 2 mM KPS, 150 mL of water, initial ethylene pressure 230−290 bar, and 1000 rpm stirring rate (pitched blade). bNumber-averaged particle diameter and polydispersity index determined by DLS. In brackets: particle diameter determined by TEM. cDetermined by the second heating cycle of DSC on the isolated bulk polymer. dDetermined by GPC at 160 °C. e Comonomer incorporation according to 13C NMR. In brackets: from ATR-IR against polyketone reference. fFormed PE (partially) deposits as bulk on the reaction mixture during reaction. gInsoluble polymer sample.
content) on cleaned coverslips and drying at room temperature for 24 h. Further drying was carried out at 70 °C under vacuum for 24 h. Films that potentially contained high-boiling comonomers were placed in 10 mL of distilled water for 5 d, washed, and dried under the same conditions.
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investigated under conditions previously established for ethylene homopolymerization (85 °C, ∼250 bar).9 Copolymerizations with more polar derivatives (HEMA, PEGA) were also conducted (Tables 1 and S1). Clay-free copolymerizations yielded colloidally stable dispersions in all cases. 13C NMR (Figures S4 and S5) and ATR-IR (Figures 1 and S6−S8)
RESULTS AND DISCUSSION
We previously showed that colloidally stable PE dispersions are obtained from a surfactant-free dispersion polymerization process with the initiator potassium persulfate, also in the absence of clay.9 This was ascribed to the ionic nature of the water-soluble initiator, which introduces negatively charged sulfate groups into the formed polymer. They, besides the adsorbed clay platelets, contribute to particle stabilization. Both clay-free particles and nanocomposites form via a coagulative particle formation mechanism in which initially formed metastable precursor particles aggregate until sufficiently high surface charge is reached (Figure S3). The clay enables heterogeneous particle nucleation on its surface, sticks to the resulting precursor particle, and retards the particle aggregation by the additional charge it contributes to colloidal stability. As a consequence, clay-stabilized polymerizations yield dispersions with smaller particles, higher particle number densities, and, due to the latter, higher yields. Assuming such particle formation also for copolymer (nanocomposite) particles, smaller particle diameters and increased polymer yield are a general indication for an additional stabilization effect of whatever nature. Notably, the polar comonomer was completely dissolved in the initial reaction mixture in all reactions, rather than forming a separate (emulsion) phase. That is, like the aqueous ethylene homopolymerization, the copolymerizations resemble dispersion polymerizations. Dispersion Copolymerization of Ethylene with (Meth-)acrylates, Acrylamides, and Vinyl Acetate. Copolymerizations of MMA and MA with ethylene were
Figure 1. Exemplary ATR-IR spectra of poly(ethylene-co-MA) copolymers with different MA contents obtained from aqueous dispersion copolymerization in the absence of clay.
analyses of precipitated polymer samples showed that, depending on the polar comonomer concentration, copolymers with 0.1−10 mol % comonomer with respect to ethylene repeat units are obtained. MA and PEGA were incorporated slightly more efficiently than MMA and HEMA. The nature of the alcohol in the (meth-)acrylic acid esters investigated did not affect the comonomer reactivity to a significant extent. The solubility of ethylene in water under polymerization conditions is [ethylene] = 0.23 M,33 and the concentrations of polar comonomers in the aqueous phase were on a similar order, namely, [polar 4272
DOI: 10.1021/acs.macromol.9b00305 Macromolecules 2019, 52, 4270−4277
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the comonomer. The broadening is especially pronounced with the hydrophilic comonomers HEMA and PEGA. Pickering-stabilized dispersion copolymerizations of ethylene with MMA and MA with Laponite yield colloidally stable clay−copolymer nanocomposite particles. Adsorbed clay platelets are visible as dark thin lines in TEM images (Figures S9 and S10) when oriented perpendicular to the TEM grid. Assuming that the added 1.5 g of Laponite exfoliates completely into disks of 25 nm diameter and a thickness of 1 nm upon dispersing it in water, its particle number density at the beginning of the reaction is approximately 2.0 × 1019 L−1. Calculation of the particle number densities of obtained nanocomposite particles yields 1.5 × 1018, 1.6 × 1018, 8.7 × 1017, 2.6 × 1018, 1.7 × 1018, and 1.0 × 1018 L−1 (entries 6, 7, 8, 14, 15, and 16, respectively). For this estimation, we assume that copolymer particles are monodisperse and spherical, with a polymer density of 0.92 g cm−3 and a neglectable influence of adsorbed clay on the measured nanocomposite particle diameter.9 This adds up to theoretically 8−23 clay particles per nanocomposite particle, which is in qualitative accordance with TEM images taking into account that also nonadsorbed clay is observed. The theoretical surface that can be covered by the clay platelets present per nanocomposite is on the same order of magnitude as the surface (determined from the average particle size) of a single nanocomposite particle. We analyzed the degree of coverage on polymer particle surfaces from ∼30 particles in TEM images (Figures S13− S16) for composites with different comonomer contents. For copolymer nanocomposites, we estimated values of 37 ± 13, 62 ± 14, 33 ± 11, and 47 ± 19% for entries 6, 8, 15, and 16 in Table 1, respectively, whereas Laponite−PE nanocomposites are covered with clay to only 29 ± 9% (entry 2, Table 1). This indicates that the higher the polar comonomer content in the copolymer the stronger the interaction of clay and copolymer nanoparticles. However, no complete coverage was observed in most samples and the particle polydispersity is increased compared to clay-free copolymerization reactions. The particle diameters of nanocomposites containing these polar comonomers are smaller than those of nanocomposites of the PE homopolymer. Additionally, the overall polymer yield is increased when MMA or MA is present during the reaction. The optimum MMA or MA concentration with respect to maximum copolymer yield was found when polar comonomer incorporations of 0.1−1 mol % are achieved. Both effects can be accounted for by an additional stabilization effect on formed particles. We postulate a similar particle formation mechanism like for the Pickering-stabilized ethylene homopolymerization9 (Figure S3) but with an improved clay−polymer particle interaction. The polymer particle hydrophilicity is increased by the presence of polar groups, which makes adsorption of clay more likely not only in the early nucleation stage of the reaction but also during the latter (particle growth) phase. This increased interaction leads to a higher degree of coverage and thus, since Laponite clay has a rather high surface charge, to an improved electrostatic stabilization of polymer particles. This retards particle coagulation so that higher particle number densities are obtained, which accelerates the overall polymerization in the following particle growth phase. It is known that both nanocomposites of MMA- and MA-containing copolymers can be synthesized via a Pickering-stabilized emulsion polymerization in which Laponite clay was found to adsorb sufficiently on such particle surfaces to stabilize them.34,35
comonomer] = 0.03−0.13 M (5−20 mmol in 150 mL of water). The copolymer samples contain relatively low contents of comonomer taking into account the monomer concentration ratio [polar comonomer]/[ethylene] and the higher reactivity in free-radical polymerization of (meth-)acrylates compared to ethylene.23 Additionally, NMR spectra show that both MMA and MA are incorporated mostly as isolated units, and only samples with high comonomer concentrations contain small amounts of (meth-)acrylate blocks, which would be expected in this copolymerization if the monomer concentrations at the loci of polymerization reaction were similar. This strongly suggests that the copolymerization takes place in the growing apolar polymer particles in which the ratio [polar comonomer]p/[ethylene]p is expected to be significantly lower than in the aqueous phase. The conversions of polar comonomer, as calculated from copolymer yield and its comonomer content, are typically 50−75% for MMA and MA. That is, while a certain compositional drift may occur during the polymerization, drastic differences in polymer composition like formation of very (meth-)acrylate-rich copolymers at the beginning and of a PE homopolymer at a later stage of the reaction are not expected, which agrees with the above microstructure analysis. We observed that the solubility of precipitated polymer samples in organic solvents was enhanced when using Bu4NBr solution instead of, e.g., brine for the precipitation of the dispersions. This was also found for dispersions obtained from the surfactant- and clay-free homopolymerization of ethylene under same conditions and is attributed to the presence of initiator-introduced sulfate groups in the polymer, which are soluble only in organic solvents when paired with a hydrophobic ammonium ion.9 The ζ-potential of the particles is −60 to −30 mV. This underlines that the particles are indeed negatively charged. For most clay-free polymerizations, the amount and type of polar comonomer did not affect the polymer yield significantly but diminished the particle size compared to the reference experiment (entry 1, Table 1). The higher the hydrophilicity of the comonomer employed (MMA, MA < HEMA, PEGA), the more pronounced this effect. When PEGA is incorporated, we additionally observe increased polymer yields compared to the ethylene homopolymerization. Particle number densities were determined9 to be 6.2 × 1016 L−1 for the clay-free homopolymerization (Table 1, entry 1) compared to 1.4 × 1017, 1.8 × 1018, 1.4 × 1017, and 4.3 × 1017 L−1 for the clay-free copolymerizations (entries 3, 9, 11, and 17, respectively). The increased particle number densities in the presence of polar comonomers indicate that copolymers behave differently in (homogeneous) nucleation and/or their primary nanoparticles aggregate to a lesser extent in the following aggregation steps. Exemplary TEM images of the spherical copolymer particles in the synthesized dispersions are depicted in Figures S9 and S10. The increased polydispersity index from DLS measurements indicates that the more comonomer is present during the reaction the broader the particle size distribution. Thermal analysis of the polymer samples reveals typical properties of LDPE synthesized under these conditions with melting points of ∼100 °C and a crystallinity of 10−30 wt % (Figure S11), which slightly decreased with increasing comonomer content, as expected. Molecular weight distributions (Figure S12) are broadened in reactions with (meth-)acrylates, and Mn is reduced, which could be caused by chain transfer reactions to 4273
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Macromolecules
Table 2. Pickering-Stabilized and Clay-Free Dispersion Copolymerizations of Ethylene with Different Ionic Comonomersa entry 1 2 3 4 5 6 7 8 9 10
comon.
comon. (mmol)
Laponite (g) 0.15
Na-MA Na-MA Na-A Na-A Na-VBS Na-VBS Na-VS VBTMAC
0.5 0.5 0.5 0.5 0.25 0.25 0.25 0.5
0.15 0.15 0.15
yield (g)
dp (nm)b
PDIb
peak Tm (°C)c
cryst. (%)c
Mw (103 g mol−1)d
Mw/Mnd
7.8 14.6 17.3 20.3 12.9 20.3 14.8 16.9 18.6 0.5
120 (120) 98 (96) 92 (95) 91 (95) 90 (93) 91 (95) 64 (61) 81 (79) 65 (61) 743 (−)
0.02 0.03 0.01 0.02 0.01 0.02 0.04 0.05 0.02 0.14
102 101 99 102 97 102 101 101 100 107
25 19 24 26 18 26 30 29 29 24
22.5
4.4
e
e
13.3
5.1
e
e
21.3
6.9
e
e
54.2
15.2
e
e
36.9 n.d.
8.0 n.d.
a Reaction conditions: 85 °C reaction temperature, 2 h reaction time, 2 mM KPS, 150 mL of water, initial ethylene pressure 230−290 bar, and 1000 rpm stirring rate (pitched blade). bNumber-averaged particle diameter and polydispersity index determined by DLS. In brackets: particle diameter determined by TEM. cDetermined by the second heating cycle of DSC on the isolated bulk polymer. dDetermined by GPC at 160 °C. ePolymer and clay not separable.
occur under these conditions (Figure S6). Since addition of rather small amounts of sodium (meth-)acrylate also resulted in an increased polymer yield and smaller particles (Tables 2 and S4), we performed copolymerizations with MMA and MA under conditions under which we can exclude that the observed additional stabilization effect arises from incorporation of small amounts of a negatively charged comonomer (Table S2). The same effect of the incorporated comonomer on particle stability in the presence of Laponite was observed even at pH 8 and a reduced reaction time of 1 h. Therefore, we conclude that the improved stabilization arises from a better interaction of clay and polymer particles and not from incorporation of a charged comonomer hydrolysis product. Also keep in mind that no additional stabilization was observed for copolymerizations with PEGA or HEMA, which are expected to possess a similar propensity for hydrolysis. Copolymerizations of ethylene with acrylamide, N-isopropylacrylamide, and vinyl acetate were investigated in clay-free and Pickering-stabilized dispersion polymerizations (Table S3). The reactions yielded colloidally stable copolymer particle dispersions in the absence of Laponite, with 0.5−3 mol % polar comonomer incorporation (Figures S17−S22). Only the Pickering-stabilized dispersion copolymerization of ethylene with acrylamide yielded colloidally stable clay−copolymer nanocomposite particles, which have a surface coverage of 39 ± 13% (Figures S16 and S20). This is also an increased degree of coverage compared to composite particles from ethylene homopolymerization. Additionally, an increased yield and a decreased particle diameter are observed, which evidences an improved polymer−Laponite interaction when acrylamide is incorporated. Copolymerizations in the presence of Laponite with vinyl acetate suffered from a low stability of the vinyl ester toward hydrolysis under polymerization conditions, as anticipated (Figure S23). Effect of Ionic Comonomers on the Dispersion Polymerization of Ethylene. Ionic olefinic compounds had a strong effect on the ethylene dispersion polymerization reaction (Tables 2 and S4). Small concentrations of sodium acrylate, sodium methacrylate, sodium vinylsulfonate, and sodium 4-vinylbenzenesulfonate, that is, salts that dissolve to form negatively charged olefins, enhance the polymer particle stabilization in terms of the increased polymer yield and decreased particle diameters. In contrast, salts that form positively charged olefins, like (vinylbenzyl)trimethylammonium chloride, lead to strong destabilization.
Note that ethylene homopolymers were found to be formed in a molten state during polymerization,9 their bulk crystallization temperature being lower than the reaction temperature of 85 °C and in addition supercooling being expected to be enhanced for crystallization in confined domains.36,37 For copolymers, with their slightly lower bulk melting and crystallization temperatures (vide supra), the same is expected. This implies that during polymerization the polymer−clay interaction is not affected by the ability of polymers to crystallize (which differs with type and degree of incorporation of comonomers), with crystallization occurring only during workup. Copolymerizations with more hydrophilic monomers like HEMA or PEGA lead to coprecipitation of clay and formed polymers during the dispersion polymerization. This led to decreased polymer yields in the presence of clay. Only a small portion of the formed copolymer remained dispersed in the aqueous phase. Thermal properties of the polymers in nanocomposite particles are similar to those of the polymers obtained without clay. However, melting points and crystallinities are slightly lower when comparing nanocomposites and clay-free copolymers with similar polar comonomer contents. This was also observed for Pickering-stabilized homopolymerization of ethylene and indicates that more amorphous, meaning more branched, polymers are formed when clay is present on the particle surface during the polymerization.9 Supposedly, the propagation rate is decelerated relative to the rate of transfer reactions promoting branching inside nanocomposite particles. The degree of crystallinity decreased for the increasing concentration of incorporated comonomers as already observed for clay-free reactions (vide supra). The polar comonomer incorporation could be determined only by ATR-IR since analysis in solution was not possible because precipitated nanocomposite materials were insoluble in common solvents even at elevated temperatures. The low solubility of clay−polymer composites is a known issue. Possibly, it is caused by shielding of the in principle soluble polymer by the insoluble silica network, which hinders the access for solvents. Attempts to separate polymeric and inorganic components by extraction with hot toluene yielded only small amounts of the polymer in the extract, which, however, were still contaminated with clay. Aqueous Laponite dispersions have a pH of ∼10. At elevated temperatures, partial hydrolysis of (meth-)acrylic esters may 4274
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Macromolecules
the clay and, thus, Pickering stabilization efficiency of Laponite. This rough approach assumes that the nanoparticle surface properties can be described with those of the bulk material. All ethylene copolymers have increased polar and slightly higher dispersive surface tensions compared to the ethylene homopolymer. Therefore, the model predicts smaller contact angles, that is, improved interaction with the likewise polar clay for all copolymers investigated. However, it indicates similar interactions of Laponite with poly(ethylene-co-AA) and poly(ethylene-co-NIPAM), which contrasts the observations from the Pickering-stabilized dispersion copolymerization. A more specific model to describe the polymer−clay interaction, which also takes into account Lewis-acid−base or hydrogenbond interactions, appears necessary to account for the differences in interaction of the Pickering stabilizer with these two copolymers.
The comonomer incorporation was too low to be detected by NMR, but ATR-IR spectra of samples synthesized in the presence of high concentrations of Na-MA and Na-A showed a vibrational band at 1550 cm−1, which is characteristic for the carboxylate group (Figure S24). However, these spectra indicate an incorporation below 0.1 mol %. This arises from the insolubility of the ionic comonomers in the growing polymer particles in which most of the polymerization takes place. However, small incorporations might occur both at the prenucleation and the particle growth stage. During the latter, growing chains can react with the comonomer in the aqueous phase and are captured by a growing polymer particle. The additional charge improves the electrostatic stabilization of the polymer particles, retards their coagulation, increases the particle number density, and leads to a faster polymerization reaction during the growth stage. In the case of a cationic comonomer, the incorporation compensates the negative charge of the initiator-introduced sulfate groups. The resulting decreased particle net charge is likely the reason for the decreased particle stabilization in this scenario. If higher concentrations of Na-A or Na-MA are present during the polymerization reaction, dispersions contain less polymer and larger particles after the same reaction time (Table S4). In general, the addition of salt increases the ionic strength and decreases the electrostatic stabilization of the particles. Apparently, at high Na-A or Na-MA concentrations, this effect overcompensates the additional stabilization effect by incorporation of the comonomer. Polymerization experiments in the presence of polyelectrolytes were performed, to exclude that Na-MA or Na-A homopolymerize in the aqueous phase and formed poly(Na-MA) or poly(Na-A) are the origin of particle stabilization or have an influence on the nucleation process (Table S5). Even small amounts of these polyelectrolytes were found to lead to destabilization, likely since they increase the ionic strength in the reaction mixture. TEM analysis of particles in the obtained dispersions reveals monodisperse and spherical nanostructures with sizes in agreement with DLS measurements (Figure S25). Pickering-stabilized reactions had to be performed with lower Laponite concentrations to avoid gelation of the clay dispersion upon addition of the ionic comonomer. This is important for the comparability of polymerizations in the absence and presence of clay since in ethylene homopolymerization the reaction mixture viscosity was found to have a tremendous effect.9 Polymerizations with Laponite clay in combination with ionic monomers yielded dispersions with a slightly increased polymer content and smaller particles than clay-stabilized polymerizations without a comonomer (entry 2). Due to the low concentrations of the clay in comparison to the high polymer contents of the dispersion, the obtained nanocomposite particles have a low degree of coverage (Figure S25). Therefore, it is likely that, besides the adsorbed clay, both the initiator-derived groups and the incorporated charged comonomer on the polymer particle surface contribute to the overall stabilization of composites. Interaction of Laponite Clay with Ethylene Copolymers. The polar and dispersive surface tensions of clay-free copolymer films were determined from experimentally observed contact angles of water and diiodomethane according to the model of Fowkes (Table S6).38 This allowed the calculation of a theoretical static contact angle of the respective material on Laponite surrounded by water to understand the role of polar functionalities in the PE on the interaction with
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CONCLUSIONS We demonstrated that colloidally stable aqueous dispersions of ethylene copolymers with 0.1−10 mol % of different (meth)acrylates, acrylamides, and vinyl acetate are accessible via a stabilizer-free dispersion copolymerization process with the initiator KPS. The particle stability arises from incorporation of charged groups introduced by the free-radical initiator. Solids contents of the copolymer dispersions are up to 9 wt %. Taking into account the similar concentrations of ethylene and comonomer and the respective copolymerization parameters, the comonomer incorporation is rather low and there is evidence that formation of comonomer blocks occurs only to a very limited extent. This indicates that the copolymerization in the heterophase system takes place mostly inside the growing polymer particles and that the solubility of ethylene in the particles is higher compared to the solubility of comonomers. The obtained copolymers have similar thermal properties like unfunctionalized LDPE particles formed under otherwise identical conditions. Ionic comonomers have a very pronounced effect on the reaction, although they are incorporated to a very small extent (
