Insights into the Network Structure of Cross-Linked Polymers

Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

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Insights into the Network Structure of Cross-Linked Polymers Synthesized via Miniemulsion Nitroxide-Mediated Radical Polymerization Ehsan Mehravar,† Amaia Agirre,† Nicholas Ballard,† Steven van Es,†,‡ Arantxa Arbe,§ Jose R. Leiza,† and José M. Asua*,†

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†

POLYMAT and Kimika Aplikatua Saila, Kimika Zientzien Fakultatea, University of the Basque Country UPV/EHU, Joxe Mari Korta Zentroa, Tolosa Hiribidea 72, E-20018 Donostia-San Sebastian, Spain ‡ Dispoltec BV, Urmonderbaan 22, 6167 RD Geleen, The Netherlands § Centro de Física de Materiales (CFM) (CSIC-UPV/EHU) and Materials Physics Center (MPC), Paseo de Manuel Lardizabal 5, E-20018 San Sebastian, Spain S Supporting Information *

ABSTRACT: The effect of reversible-deactivation radical polymerization on the structure of the network formed by copolymerization of mono- and divinyl monomers is studied. The nitroxide-mediated radical copolymerization (NMP) of butyl methacrylate and ethylene glycol dimethacrylate, initiated with the alkoxyamine 3-(((2-cyanopropan-2yl)oxy)cyclohexylamino)-2,2-dimethyl-3-phenylpropanenitrile (Dispolreg 007) in an aqueous miniemulsion was used as a case study. Combination of asymmetric-flow fieldflow fractionation and small-angle X-ray scattering clearly showed that NMP led to a more homogeneous network structure formed by regions of relatively open networks connected with short polymer chain segments, whereas that of the polymer formed by free radical polymerization consists of densely cross-linked regions linked by long polymer chain portions.

1. INTRODUCTION Cross-linked polymers are used in a broad range of applications, including superabsorbent materials, ion-exchange resins, dental restorative materials, chromatography packings, cosmetics and pharmaceuticals, drug-delivery systems, artificial organs, sensors, optics, and electronics.1 Conventional free radical polymerization (FRP) of monovinyl monomers in the presence of a rather small amount of divinyl monomer is one of the simplest routes for the preparation of cross-linked polymers. However, due to the inherent features of FRP (relatively slow initiation, fast chain propagation, and high termination rate), the polymer network synthesized through this route has a heterogeneous structure.2 This means that some sections may be very tightly cross-linked (high cross-link density), while other sections could exhibit very loose networks. This poses a problem in applications where specific and well-controlled material properties are required (e.g., superabsorbents, contact lenses). Hence, production of a more uniform network is generally desirable.1−3 The advent of reversible-deactivation radical polymerization (RDRP)4 has made it possible to synthesize polymers with unprecedented control over the macromolecular structure.5,6 Attempts to extend this concept to cross-linked polymers have been reported using a range of RDRP techniques such as atom transfer radical polymerization (ATRP),2,7−9 reversible addi© XXXX American Chemical Society

tion−fragmentation chain transfer polymerization (RAFT),10−12 and nitroxide-mediated radical polymerization (NMRP).3,13−30 The most extensively studied cross-linked system (both experimentally and by simulation) is the 2,2,6,6tetramethylpiperidinyl-1-oxy (TEMPO)-mediated copolymerization of styrenic monomers and divinyl monomers in bulk, 13,15,21,30 solution, 3,14,16−19,21,24,27−29 miniemulsion,20−22,25,26 or suspension.23 In general, these works reported a lower and well-controlled rate of polymerization, delayed gelation, and higher swelling than in the counterpart experiments carried out by FRP. Although in almost all of the experimental studies it has been claimed that the polymer networks obtained through RDRP were more homogeneous than in FRP, this idea is mostly based on mechanistic speculations of how polymer networks are produced. On the other hand, this idea has been challenged by researchers arguing that the only effect of RDRP is to delay gelation, without any significant effect on the homogeneity of the polymer network.3,31 Therefore, a more detailed study providing experimental evidence is required to clarify this issue. Received: August 1, 2018 Revised: November 2, 2018

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DOI: 10.1021/acs.macromol.8b01648 Macromolecules XXXX, XXX, XXX−XXX

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Two cycles were used to obtain homogeneous miniemulsions with small droplet sizes. The miniemulsion was transferred to a 1 L glass reactor vessel equipped with a platinum resistance thermometer, condenser, nitrogen inlet, sampling tube, and stainless steel anchor impeller rotating at 150 rpm. It is worth mentioning that the NMP miniemulsion contained the alkoxyamine initiator, whereas the miniemulsion used for free radical polymerization was devoid of AIBN. The miniemulsions were purged with nitrogen and heated to the reaction temperature (70 °C for FRP and 90 °C for NMP). For FRP, the AIBN solution (solution of AIBN in 3 g of BMA monomer) was then added in one shot. The FRP and NMP miniemulsion experiments were carried out for 1.5 and 8 h, respectively. In addition to cross-linked copolymers, a linear poly(BMA) (without using EGDMA in the formulation) was synthesized by NMP for comparison purposes in SAXS analysis. Overall conversion was determined gravimetrically. The samples were dried at 60 °C, first overnight at atmospheric pressure and then for 24 h under vacuum. Analysis and Characterization. Monomer droplet and particle sizes were measured by dynamic light scattering in a Zetasizer Nano Z (Malvern Instruments). The samples were prepared by dilution of the latex in distilled water. The values given are z-average values obtained through cumulants analysis. The equipment was operated at 20 °C, and the values reported are the average of two repeated measurements. The molar mass distribution and the distribution of the radius of gyration of the whole polymer as well as gel content and swelling degree were measured by asymmetric-flow field-flow fractionation (AF4, Wyatt Eclipse 3) with multiangle light scattering (MALS) and refractive index (RI) detectors and using THF as the solvent. The setup consisted of a pump (LC-20, Shimadzu) coupled to a DAWN Heleos multiangle light scattering laser photometer (MALS, Wyatt) equipped with a He−Ne laser (λ = 658 nm) and an Optilab Rex differential refractometer (λ = 658 nm) (RI, Wyatt Technology). In AF4, the separation is based on the interplay between the flows of the carrier and the Brownian motion of the macromolecules occurring in an open channel.36 The main advantage of AF4 is that due to the lack of stationary phase very large macromolecules can be analyzed. The data collection and treatment were carried out by ASTRA 6 software (Wyatt Technology). The samples were prepared by dispersing the latexes directly in THF (5 mg of polymer latex in 1 mL of THF). The molar mass was calculated from the RI/MALS data using the Debye plot (with second-order Berry formalism). In addition to the molar mass, the distribution of the radius of gyration of the swollen polymer gels was measured by the angular dependence of the MALS detector. The z-average radius of gyration of the nanogels (Rz, nm) was used to characterize the swelling degree of the nanogels (Sw, dimensionless), which was calculated as the ratio of the volume of the swollen nanogel estimated from Rz and the volume of the polymer estimated from the weight-average molecular weight (M̅ w) assuming a polymer density ρp = 1.1 g cm−3.

In this work, we provide this evidence using the batch nitroxide-mediated miniemulsion copolymerization of butyl methacrylate and ethylene glycol dimethacrylate. A novel alkoxyamine (3-(((2-cyanopropan-2-yl)oxy)cyclohexylamino)2,2-dimethyl-3-phenylpropanenitrile, Dispolreg 007), which has been readily synthesized on a large scale at low cost and is able to control the polymerization of methacrylates in both organic and aqueous media,32−35 is used. The evolution of the polymer microstructure during polymerization was compared with that obtained in a conventional free radical polymerization. The evolution of the molar mass distribution of the whole polymer (i.e., including the gel fraction) determined by asymmetric-flow field-flow fractionation (AF4) and small-angle X-ray scattering (SAXS) was used to determine the structure of the polymer network.

2. EXPERIMENTAL SECTION Materials. n-bButyl methacrylate (BMA; >99%, Sigma-Aldrich) and ethylene glycol dimethyl acrylate (EGDMA; Aldrich) were used without purification. Alkyl diphenyl oxide disulfonate (DOWFAX 2A1) as the anionic emulsifier and hexadecane (HD; Aldrich) as the costabilizer were used to stabilize the miniemulsions. 2,2′-Azobis(2methylpropionitrile) (AIBN; Aldrich) and (3-(((2-cyanopropan-2yl)oxy)cyclohexylamino)-2,2-dimethyl-3-phenylpropanenitrile) (Dispolreg 007, Dispoltec) were used as oil-soluble thermal initiators for FRP and NMP, respectively. The chemical formula of the alkoxyamine is shown in Scheme 1. Deionized water was used as the polymerization medium.

Scheme 1. Chemical Formula of the Dispolreg 007 Alkoxyamine Initiator Used in the NMP System

Miniemulsion Polymerization. Miniemulsions were prepared using the formulation in Table 1. The organic phase (the mixture of

Table 1. Formulation Used To Synthesize BMA/EGDMA Copolymers in Both Nitroxide-Mediated and Free Radical Miniemulsion Polymerizations (Solids Content 20 wt %) component

amount (g)

BMA EGDMA HD alkoxyamine (or AIBN) DOWFAX 2A1 water

120 2.52a 9b 1.44c (or 0.348 for AIBN) 2.7d 480

Sw =

4πR z 3NAρp 3M̅ w

× 10−21

(1)

The distribution of the radius of gyration also provided information on the development of the networks produced in both types of polymerization. Small-angle X-ray scattering (SAXS) experiments were carried out in Rigaku three-pinhole PSAXS-L equipment. The MicroMax-002+ Xray generator system is composed of a microfocus X-ray source, which produces Cu Kα transition photons of wavelength λ = 0.1542 nm. The flight path and the sample chamber in this instrument are under vacuum. The scattered X-rays are detected on a two-dimensional multiwire X-ray detector (Gabriel design, 2D-200X) offering a 200 mm diameter active area with ca. 200 μm resolution. After azimuthal integration, the scattered intensities were obtained as a function of the scattering vector Q = 4πλ−1 sin θ (2θ = scattering angle). Reciprocal space calibration was done using silver behenate as the standard. The

a

Concentration of 1.45 mol % based on monomers. bConcentration of 7.5 wt % based on monomers. cDp,target= [BMA]/[Dispolreg 007] = 200. dConcentration of 2 wt % based on monomers. monomers, HD, and alkoxyamine initiator (Dispolreg 007) in the case of NMP) was added to the aqueous phase (the mixture of DOWFAX 2A1 and total amount of deionized water) under mechanical agitation. The coarse emulsion was sonicated by using a Branson Sonifier 450 (amplitude 70% and 50% duty cycle) over 10 min under magnetic stirring and in an ice bath to avoid overheating. Then, the miniemulsions were fed into a Niro Soavi laboratory homogenizer, whose first valve was set to 6 × 107 Pa and the second to 6 × 106 Pa. B


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Macromolecules sample to detector distances were 0.5 and 2 m, covering a Q-range between 0.008 and 0.9 Å−1. The samples were prepared by swelling the polymer films in THF (a thin film was introduced in a capillary and then the capillary filled up with THF), and the measurements were performed at room temperature. The mechanical properties of films cast from dispersions were studied by dynamic-mechanical thermal analysis (DMTA). The measurement was carried out using DMTA (Triton 2000 DMA, Triton Technology, Ltd.) equipment. The copolymer samples of 7 × 10 × 0.7 mm3 were obtained from dried copolymers using a hydraulic hot press at 150 °C for 15 min under a pressure of 100 bar and then cooled to room temperature. A single cantilever tension (bending) geometry was used. The real (storage modulus, E′) and imaginary (loss modulus, E″) components of the complex shear modulus E* = E′+ iE″ and tan δ = E″/E′ (mechanical loss) were measured from −50 to +150 °C at 1 Hz. The heating rate was 4 °C/min. During the experiment, a sinusoidal strain was applied and the response of the sample (the stress necessary to maintain the deformation) registered.

order kinetic plot for the NMP reaction suggests a wellcontrolled polymerization, in contrast to that of FRP, which shows an acceleration in rate at the 10−15% conversion range (Figure 2b) due to the gel effect. The slight increase in the polymerization rate that can be observed for the NMP system has previously been observed in both miniemulsion and suspension polymerization34,35 and has been attributed to the relatively slow rate of decomposition of Dispolreg 007 at this temperature. The samples of the latex withdrawn from the reactor were put directly in THF under vigorous agitation (to avoid particle coalescence), and the mixing was continued for 12 h before analysis by AF4/MALS/RI. Figure 3 presents the evolution of the molar mass distribution during the polymerization for the NMP system (RI chromatogram vs elution time plots for NMP system are shown in Figure S1 in the Supporting Information). It can be seen that, at 16% conversion, the molar mass distribution (MMD) presented a single peak with an average molar mass of about 2 × 104 g mol−1, showing that no gel had been formed. At higher conversions (18−37%), a new peak appeared at higher molar mass (about 3 × 107 g mol−1), and the size of the small molar mass peak decreased and moved toward higher molar masses. This indicates that chains are growing and that cross-linking reactions started to take place. A significant change occurred between 37% and 44% conversion, with the low molar mass peak virtually disappearing and most of the polymer forming a high molar mass peak ((2−3) × 108 g mol−1). As conversion increased further, the average molar mass of the gel polymer increased, reaching approximately the theoretical maximum assuming a single cross-linked chain per particle. It can be calculated that the maximum molar mass in a particle of 172 nm diameter (average size of the particles) is about 1.8 × 109 g mol−1, which is smaller than those of some of the nanogels detected in Figure 3. The distribution of particle sizes (PDI (polydispersity index) = 0.06) with particles significantly larger than the average size accounts for these large nanogels. The gel content of the samples was estimated from the molar mass distributions (the fraction of polymer with molar mass greater than 2 × 107 g mol−1 was considered as a gel in an admittedly arbitrary definition) and plotted in Figure 4a. The results show that the gelation point could be placed at 37−44% conversion. For the sake of comparison, the evolution of the MMD during the free radical miniemulsion polymerization of the same monomer system was determined. Figure 5 shows that very high molar masses were obtained at conversions as low as

3. RESULTS AND DISCUSSION The evolution of the particle size during NMP and FRP is given in Figure 1. It can be seen that the final particle sizes of

Figure 1. Evolution of the particle size during the miniemulsion copolymerization of BMA/EGDMA for both NMP (90 °C) and FRP (70 °C).

the latexes were almost the same as the initial droplet size of the miniemulsions, indicating an efficient droplet nucleation. Moreover, it can be observed that the particle size of the latex synthesized by NMP (172 nm) was higher than that of the FRP latex (142 nm), which can be attributed to the hydrophobicity of Dispolreg 007 and the different temperatures used in the polymerizations. Figure 2a shows that the NMP system proceeded smoothly to high conversions whereas the FRP system was fast, and full monomer conversion was achieved after 50 min. The first-

Figure 2. (a) Total conversion vs time and (b) first-order rate plots for nitroxide-mediated (90 °C) and free radical (70 °C) miniemulsion copolymerization of BMA and EGDMA. C


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Figure 3. Evolution of the molar mass distribution in the batch miniemulsion NMP of BMA and EGDMA.

Figure 4. (a) Gel content, (b) weight-average molar mass of the whole polymer, and (c) swelling degree evolution in the nitroxide-mediated (NMP) and free radical (FRP) miniemulsion copolymerization of BMA/EGDMA.

3%. Even at this low conversion, a significant fraction of gel was measured (Figure 4a) (RI chromatogram vs elution time plots for the FRP system are shown in Figure S1 in the Supporting Information). Above 16%, only the very high molar mass peak was detected. The molar mass of this peak increased with conversion as a result of the cross-linking reactions, which is in good agreement with the prediction of the mathematical modeling for the S/DVB system.30 Figure 4 summarizes the comparison between NMP and FRP. It can be seen that the gelation point is substantially delayed in NMP. This relates to the manner in which the molar mass develops in NMP, where in the initial phase of

reaction the chain lengths are short and therefore the probability of having EGDMA units able to cross-link is small. For a detailed description of the mechanisms involved in the nitroxide-mediated copolymerization of vinyl and divinyl monomers, the reader is referred to the works dealing with mathematical modeling of these processes (e.g., refs 24 and 30). Figures 3, 4b, and 5 show that, up to 40−50% conversion, the weight-average molar mass was smaller for NMP than for FRP, but at higher conversions the NMP molar mass was higher, likely due to the larger particle size in the NMP system. The evolution of the swelling degree of the nanogels (Sw) is presented in Figure 4c. It can be seen that, for the same D


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than for the NMP polymer, whereas they presented much closer values at higher conversions. Similar results have been reported for polymers prepared by NMP and FRP of styrene and divinylbenzene in aqueous microsuspension.23 Likely, the differences between NMP and FRP are due to the fact that the FRP samples were formed only by nanogels (Figures 4 and 5), whereas for NMP formation of nanogels occurred above 44% conversion. In the rubbery state (Figure 7b), E′ was higher for FRP than for NMP, and both increased with conversion, likely due to the increase in the molar mass of the nanogels in both cases. At very high conversion, E′ for NMP is still lower than that of FRP, confirming the presence of more cross-linked domains in the FRP sample. The higher value of tan δ for FRP in the whole range of conversion further supports this hypothesis. To obtain more information about the internal structure of the gel, the final samples were analyzed by small-angle X-ray scattering (SAXS). Figure 8 shows the SAXS results of the two cross-linked films swollen in THF. Results for poly(BMA) synthesized without cross-linker (linear polymer) using NMP are also included for comparison purposes. The intensity scattered in the SAXS region is a signature of the presence of heterogeneities in the scattering length density at the length scales probed by this technique. A pronounced low-Q contribution appears in the case of the gels containing crosslinked chains, which is absent in the solution of non-crosslinked chains. This extra intensityand thus the degree of heterogeneity at such large length scalesis clearly more prominent in the case of the FRP sample than in the NMP system. In addition, for all samples, we observe the presence of a peak centered at about 0.5 Å−1. We note that a peak in this range has also been reported in the structure factor of bulk samples of PBMA and is attributed to correlations between nanodomains formed by the lateral groups.40 A pseudo-Voigt function

Figure 5. Evolution of the molar mass distribution during the batch miniemulsion FRP of BMA and EGDMA.

conversion, swelling was higher for NMP; namely, the nanogels formed by FRP were more cross-linked. Analysis of the evolution of the distribution of the radius of gyration shed further light on the differences of the network structures formed in each polymerization. Figure 6 shows that, for both NMP and FRP, the radius of gyration decreased as the conversion increased. Taking into account that in both cases the molecular weights increased with the conversion (Figures 3 and 5), this clearly demonstrates that cross-linking increased during the process. On the other hand, the distribution of the radius of gyration for NMP was narrow, indicating a rather homogeneous cross-linking, whereas the broad distributions obtained for FRP were the consequence of a heterogeneous network structure. To gain further insight into the structure of the polymer networks, the specimens prepared from cast films of the NMP and FRP dispersions were analyzed by linear rheology.37−39 The evolutions from −50 °C (glassy state) to +150 °C (rubbery state) of the storage modulus (E′) and tan δ with the monomer conversion for NMP and FRP are given in Figures S2 and S3 of the Supporting Information. The results are summarized in Figure 7. It can be seen that, in the region of glassy to rubbery state transition (25 °C), E′ linearly increased with the conversion for NMP, whereas it was roughly constant for FRP. At low/intermediate conversion (60%), there were still polymer chains not incorporated into the nanogels. This led to the open structure shown in Scheme 2. On the other hand, in FRP, the lifetime of each chain is very short, and long chains containing plenty of EGDMA units are formed from the very beginning. This led to very compact network structures that were interconnected by long polymer chains. The network structures built from the SAXS measurements agree with the lower value of the storage modulus of the NMP samples in the rubbery state.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01648.

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RI chromatogram vs elution time plots for NMP and FRP systems and evolution of the storage modulus and tan δ with conversion for the BMA/EGDMA copolymers synthesized by NMP and FRP (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

4. CONCLUSIONS In this work we report the effect of reversible-deactivation radical polymerization on the homogeneity of the network structure formed by copolymerization of mono- and divinyl monomers. The case is made by comparing the polymer made by the nitroxide-mediated radical copolymerization of butyl methacrylate (BMA) and ethylene glycol dimethacrylate (EGDMA) using the alkoxyamine 3-(((2-cyanopropan-2yl)oxy)cyclohexylamino)-2,2-dimethyl-3-phenylpropanenitrile (Dispolreg 007) in aqueous miniemulsion with that obtained by regular free radical miniemulsion polymerization (FRP) of BMA/EGDMA as a reference. Nitroxide-mediated polymerization (NMP) delays the onset of gelation and lowers the molar mass of the polymer (especially at lower conversions) in comparison with FRP. At all conversions, swelling was higher for NMP; namely, the nanogels were less cross-linked. In the glassy state (25 °C), the storage modulus (E′) of the NMP polymer increases with the conversion while that of the FRP polymer is constant, whereas, in the rubbery state, E′ increases with conversion for both polymers. At full conversion, the E′ values of the two polymers are similar in the glassy state and that for the NMP polymer is slightly lower in the rubbery state. On the other hand, tan δ is lower for NMP, which further supports the presence of more cross-linked domains in FRP.

Ehsan Mehravar: 0000-0002-4345-653X Steven van Es: 0000-0002-9395-1096 Arantxa Arbe: 0000-0002-5137-4649 Jose R. Leiza: 0000-0001-9936-7539 José M. Asua: 0000-0002-7771-1543 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Diputación Foral de Gipuzkoa, University of Basque Country UPV/EHU (UFI 11/56), Basque Government (Grants GVIT373-10 and GVIT654-13), and MINECO (Grants CTQ 2016-80886-R, CTQ 2017-87841-R, and MAT201563704-P) are gratefully acknowledged for their financial support. N.B. acknowledges the financial support obtained through the postdoctoral fellowship Juan de la Cierva Incorporación (Grant IJCI-2016-28442) from the Ministry of Economy and Competitiveness of Spain.

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