Incorporating Cellulose Nanocrystals into the Core of Polymer Latex

Aug 1, 2018 - ... for controlling the location of CNCs in latex-based nanocomposites and may extend the use of CNCs in commercial adhesives and coatin...
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Letter Cite This: ACS Macro Lett. 2018, 7, 990−996

Incorporating Cellulose Nanocrystals into the Core of Polymer Latex Particles via Polymer Grafting Stephanie A. Kedzior,*,†,‡ Michael Kiriakou,† Elina Niinivaara,† Marc A. Dube,́ § Carole Fraschini,∥ Richard M. Berry,⊥ and Emily D. Cranston† †

Department of Chemical Engineering, McMaster University, 1280 Main Street West, Hamilton, Ontario, Canada, L8S 4L7 Department of Chemical and Biological Engineering, Centre for Catalyst Research and Innovation, University of Ottawa, 161 Louis Pasteur Pvt., Ottawa, Ontario, Canada, K1N 6N5 ∥ FPInnovations, 570 Boulevard St. Jean, Pointe-Claire, Quebec, Canada, H9R 3J9 ⊥ CelluForce Inc., 625 Président-Kennedy Avenue, Montreal, Quebec, Canada, H3A 1K2

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S Supporting Information *

ABSTRACT: Surface-initiated atom transfer radical polymerization was used to graft hydrophobic poly(butyl acrylate) from cellulose nanocrystals (CNCs) resulting in compatibilized CNCs that were successfully incorporated inside the core of polymer latex particles. CNCs are anisotropic nanoparticles derived from renewable resources and have potential as reinforcing agents in nanocomposites. However, challenges due to the incompatibility between cellulose and hydrophobic polymers and processing difficulties, such as aggregation, have limited the performance of CNC nanocomposites produced to date. Here, CNCs were incorporated into the miniemulsion polymerization of methyl methacrylate by adding polymer-grafted CNCs to the monomer phase. A poly(methyl methacrylate)-CNC nanocomposite latex was subsequently produced in situ, whereby polymer-grafted CNCs (with optimized graft length) were located inside the latex particles, as shown by transmission electron microscopy. This work provides a method for controlling the location of CNCs in latex-based nanocomposites and may extend the use of CNCs in commercial adhesives and coatings.


CNCs are stiff rod-shaped nanoparticles derived from renewable resources such as wood pulp and cotton.15 As a result of the shift toward using greener chemistries and the drive to replace synthetic materials with renewable alternatives, incorporating CNCs into nanocomposites has been the focus of much work to date.16,17 CNCs are most commonly produced by hydrolysis (or oxidation18,19), and when sulfuric acid is used, the amorphous cellulose regions are degraded and crystalline nanoparticles with an abundance of surface hydroxyl groups and some anionic sulfate half ester groups are isolated.20 The sulfate half ester groups impart good colloidal stability to CNCs, which make processing and handling straightforward.21 Due to the primarily hydrophilic nature of CNCs, unmodified CNCs may be easily incorporated into hydrophilic polymer matrices such as starch/glycerol,22 poly(ethylene oxide),23 and carboxymethylcellulose.24 Conversely, adding CNCs to hydrophobic polymers leads to nanoparticle aggregation, processing difficulties, and weak mechanical

olymer latexes are stable dispersions of nano- or microscale polymer particles that are used in applications such as paints, adhesives, protective coatings, cosmetics, and toners. Typically, an aqueous latex dispersion is cast as a film in order to form a uniform coating upon the evaporation of water.1 Polymer latexes can be composed of low glass transition (Tg) polymers, such as poly(butyl acrylate) (PBA), and high Tg polymers, such as poly(methyl methacrylate) (PMMA), and have been widely investigated and used in commercial products.2,3 Latexes are prepared via emulsion or miniemulsion polymerization, whereby a water insoluble monomer is dispersed in water using a stabilizing agent (typically surfactant), and the polymerization is initiated in the water phase (emulsion polymerization) or within the monomer droplets (miniemulsion polymerization).4 To enhance the mechanical properties of films prepared from latex dispersions, fillers or reinforcing agents can be added. Fillers such as nano-ZnO,5,6 nano-TiO2,7 nano-Al2O3,8 nanoSiO2,9 and anisotropic nanoparticles, such as nanoclays,10 carbon nanotubes,11 and cellulose nanocrystals (CNCs),12−14 have shown promise to provide latex films with improved thermal, mechanical, and adhesive properties. © XXXX American Chemical Society

Received: May 1, 2018 Accepted: July 31, 2018


DOI: 10.1021/acsmacrolett.8b00334 ACS Macro Lett. 2018, 7, 990−996


ACS Macro Letters

Figure 1. Schematic showing the modification of unmodified CNCs with ATRP initiator groups to prepare BiB-CNCs, and the SI-ATRP of BA to prepare CNC-g-PBAn, where n is the target degree of polymerization.

grafted CNCs were prepared via SI-ATRP as shown in Figure 1 and following the protocol described in the Supporting Information. CNCs were first modified with α-bromoisobutyryl bromide (BiB) to immobilize surface initiating groups, resulting in 3.5 wt % Br (determined via elemental analysis), which correlates to about 1 chain per 2 nm2 on the CNC surface, comparable to other reports of SI-ATRP from CNCs.45,48−51 This initiator modification step has been shown to result in a decrease in thermal stability and a slight decrease in crystallinity.46 Butyl acrylate was subsequently polymerized from the surface of BiB-CNCs, and elemental analysis postgrafting determined that the Br content remained at 3.5 wt %, implying that the same number of active sites were present after polymer grafting; therefore, the grafted polymer chains are in fact end-functionalized with Br, as expected when grafting via ATRP. We chose a target degree of polymerization (DP), that is, graft length, of 50 and 200, such that the polymer chains grafted from the CNCs had a target molecular weight of 6.4 and 25.6 kDa, respectively. Characterization of the free polymer (polymerized in solution by “sacrificial initiator” and collected via centrifugation, and shown previously to be a suitable comparison to the polymers grafted from the CNC surface43) determined that good control was achieved (PDI = 1.2 for DP 50 and PDI = 1.1 for DP 200) and a MWn of 6.8 and 21.6 kDa was measured via GPC (Supporting Information, Table S1). Furthermore, polymer grafting from the CNCs was supported by ATR-FTIR of the centrifuged and purified CNCg-PBA50 and CNC-g-PBA200 and is shown in the Supporting Information, Figure S1. To produce latex particles with CNCs inside the core, the polymer-grafted CNCs needed to be dispersed in the MMA monomer phase of the miniemulsion polymerization prior to emulsification and initiation. Thus, the colloidal stability of CNCs in MMA and their preference for the water or monomer phase was investigated. Monomer suspensions containing 0.5 wt % CNC-g-PBA50 or 0.5 wt % unmodified CNCs were prepared and subsequently water was added to achieve the same monomer/water ratio as in the miniemulsion polymerization reactions. MMA monomer without CNCs was a clear liquid, as shown in Figure 2a, while a suspension of unmodified CNCs in MMA was white and appeared unstable, as aggregates quickly settled to the bottom of the vial (Figure 2b). In comparison, a suspension of CNC-g-PBA50 in MMA (Figure 2c) appeared slightly yellow in color (since dried CNC-gPBA50 is yellow) and was more stable than unmodified CNCs

properties. An alternate route to well-dispersed CNC composites is to add CNCs to the water phase of latex dispersions by blending postsynthesis25−32 or by addition in situ12−14,33−36 during emulsion polymerization. The resulting nanocomposite latex films have improved mechanical and adhesive properties,13,14 but in all existing work, the CNCs are located outside of the polymer particles, that is, either dispersed in the water phase or at the polymer−water interface. The novelty in the work presented here is that CNCs can be encapsulated inside the latex particles by compatibilizing CNCs with the monomer phase. We predict that the new nanocomposite latexes and our ability to control the CNC location can further improve mechanical and adhesive properties and extend applications of CNCs to more hydrophobic polymer matrices and new products. To compatibilize CNCs with hydrophobic monomers, polymers, and solvents, surface modification via noncovalent or covalent modifications is required. Noncovalent modifications can exploit the anionic surface charge on CNCs to electrostatically bind cationic surfactants in order to coat the CNCs and allow the hydrophobic tails to act as interfacial compatibilizers.37−39 However, in the synthesis of polymer latexes, the use of surfactants to modify CNCs is complicated by the presence of other surfactants required to stabilize the monomer droplets. Other methods of compatibilizing CNCs are via small molecule functionalization or polymer grafting,40 such as free radical polymerization, or surface-initiated atom transfer radical polymerization (SI-ATRP), used here. SIATRP has previously been employed to graft hydrophobic polymers such as poly(styrene),41−44 poly(methacrylate),45,46 PMMA,47 and copolymers of poly(MMA-co-BA),48 among others. The reported polymer-grafted CNCs generally have increased contact angles and, thus, better compatibility and reinforcing potential, for example, in PMMA matrices.44 Herein, we use SI-ATRP to compatibilize CNCs with the methyl methacrylate (MMA) monomer phase in a miniemulsion polymerization by grafting hydrophobic PBA from CNCs. While we used polymer grafting to “hydrophobize” CNCs, we believe that other covalent and noncovalent hydrophobic modification routes are likely suitable, such that if CNCs are nonaggregated and compatible with the monomer phase, they remain inside the monomer droplets as they are converted to latex particles during polymerization. The “living” nature of SI-ATRP allows for control over the polymer graft length and density.42 In this work, polymer991

DOI: 10.1021/acsmacrolett.8b00334 ACS Macro Lett. 2018, 7, 990−996


ACS Macro Letters

diffusion between droplets, was omitted, providing a simple “proof-of-concept” system where the effect of polymer-grafted CNCs on the polymerization could be easily interpreted. We have also demonstrated before that when CNCs act as Pickering stabilizers on the outside of the monomer droplets (in miniemulsion polymerization) that there are minimal differences between systems with and without hydrophobic agents.36 Future work will involve the use of a hydrophobic agent as well as determining the effect of the latex monomer type on the incorporation of polymer-grafted CNCs. PMMA latexes with unmodified CNCs, CNC-g-PBA50, and CNC-g-PBA200 added in situ to the miniemulsion polymerization were successfully produced giving uniform latex dispersions and exhibiting minimal coagulation in all cases. The size of the PMMA latex particles with CNC-g-PBA50 were larger than those without CNCs, and were also larger than those with unmodified CNCs and CNC-g-PBA200; however, overall, the hydrodynamic diameters (measured by dynamic light scattering, Table 1) remained nanosized with similar

Figure 2. MMA monomer with (a) no CNCs, (b) 0.5 wt % unmodified CNCs, and (c) 0.5 wt % CNC-g-PBA50. Water was then added to MMA-CNC suspensions, the mixture was then vortexed and allowed to phase separate showing (d) MMA phase on top of the water phase with no CNCs added, (e) the presence of unmodified CNCs in the bottom water phase, and (f) the presence of CNC-gPBA50 in the top MMA phase.

Table 1. Physical Properties of PMMA Latexes with No CNCs, Unmodified CNCs, and Polymer-Grafted CNCs Added to the MMA Phase before Polymerization

as no precipitation was visible. Water was then added to the MMA suspensions and the mixtures were vortexed and left to phase separate such that the polymer-grafted and unmodified CNCs were allowed to partition to their preferred phase. A photo was taken after 5 min of resting (Figure 2d−f), which shows that unmodified CNCs had a preference for the bottom water phase, as indicated by the translucent blue tint in the water (Figure 2e), while the visible majority of CNCs grafted with PBA (Figure 2f) did not partition into the water phase. The grafted CNCs partitioned into the MMA phase but may have created a “gradient” third phase that partially settled near the bottom of the MMA phase (above the water phase). When all three phases were imaged with optical microscopy, no presence of an oil-in-water emulsion was observed, and therefore, the polymer-grafted CNCs were not partitioning to the MMA/water interface and were not acting as Pickering stabilizers. Furthermore, when the MMA phase was weighed it was determined that the majority of CNC-g-PBA50 remained in the MMA phase, while a small fraction fell to the water phase. As the contact angle of the CNC-g-PBA50 was determined to be 50° (Supporting Information, Table S1), it is not surprising that there is a fraction of CNC-g-PBA50 that may fall into to the water phase. However, these observations suggest that during the sonication step required to prepare the emulsion (prior to polymerization), the majority of polymer-grafted CNCs remained dispersed in the monomer phase and were not likely to be present in the water phase, which is essential for incorporating the polymer-grafted CNCs inside the polymer latex particles. PMMA nanocomposite latexes were then prepared by miniemulsion polymerization, whereby the MMA monomer containing 0.5 wt % CNCs was dispersed in the water phase using the anionic surfactant sodium dodecyl sulfate (SDS). We have previously shown that SDS does not interact with CNCs.36 It is also important to note that the surfactant concentration used was below the critical micelle concentration. This was chosen to produce larger monomer droplets which we anticipated could help with incorporating CNCs into the monomer droplets and hopefully the polymerized latex particles. A hydrophobic agent, typically added to compartmentalize the monomer droplets and reduce monomer

CNC type none unmodified CNCs CNC-g-PBA50 CNC-g-PBA200

latex size (nm)

latex size polydispersity

latex zeta potential (mV)

latex Tg (°C)

204 ± 16 180 ± 13

0.36 0.28

−30 ± 8 −21 ± 6

100 108

248 ± 9 162 ± 7

0.39 0.28

−15 ± 6 −36 ± 6

103 103

polydispersity. The increase in latex diameter was expected, as “fully” encapsulating CNCs (average length 183 nm52) would require a PMMA latex with a larger diameter than that of the base case. Furthermore, the zeta potential of all PMMA latexes remained negative, confirming the presence of anionic surfactant at the surface of the latex particles (and not polymer-grafted CNCs) and that good colloidal stability of the PMMA particles was maintained throughout the reaction (Table 1). Indirectly, the more negative zeta potential for the latex with CNC-g-PBA200, compared with CNC-g-PBA50, may indicate more CNCs on the outside of the latex particles as CNCs are also anionic and contribute to the negative zeta potential if not incorporated into the polymer particles. DSC was used to measure the Tg of the PMMA nanocomposite latexes (Table 1). A shift in Tg would imply a change in the polymer mobility, and tethering of PMMA polymers within the latex to the polymer-grafted CNCs. As the polymer-grafted CNCs are end functionalized with Br, there was a possibility of chain extension or chain transfer via radical abstraction of the Br. However, no significant difference in Tg was observed for latexes with CNC-g-PBA50 or CNC-g-PBA200, indicating that the polymer-grafted CNCs are not necessarily tethered or covalently bound to the PMMA latex, or that polymer-grafted CNCs are not added in high enough quantities to detect a significant difference in Tg. The nanocomposite PMMA latex particles were imaged by TEM to determine their size and morphology and to identify the location of the CNCs (Figure 3). In the PMMA latex without CNCs, only spherical nanoparticles with some aggregation were observed (Figure 3a). With the addition of unmodified CNCs into the MMA phase, spherical PMMA particles were observed (similar to the base case), along with 992

DOI: 10.1021/acsmacrolett.8b00334 ACS Macro Lett. 2018, 7, 990−996


ACS Macro Letters

individual latex particles as in the latex containing CNC-gPBA50 as shown in the left most latex particle in Figure 3c. Additional imaging of latexes filtered through 1 μm pore size filters supported that CNC-g-PBA50 was incorporated inside the latex particles but CNC-g-PBA200 was not. Supporting Information, Figure S3a, shows that rod-shaped CNCs were present alongside spherical PMMA nanoparticles with CNC-gPBA200 after filtering with a 1 μm filter. The inability to incorporate polymer-grafted CNCs at DP 200 suggests that at higher DP, the polymer-grafted CNCs are too large or too aggregated (therefore lacking the high aspect ratio that unmodified CNCs possess) to be fully incorporated. In fact, based on previous work, it is estimated that CNC-g-PBA50 is only 60% CNC by mass, while CNC-g-PBA200 is only 25% CNC by mass (based on thermal gravimetic analysis on CNCg-PMA at approximately the same DP46). Furthermore, by rough calculation, PBA chains with DP 50 have an end to end distance (for a fully extended chain) of 15 nm, while PBA chains with DP 200 have an end to end distance of 62 nm. This increase could result in entanglement of polymer chains on neighboring CNCs, which likely leads to lose CNC-gPBA200 agglomerates that are too large to be fully encapsulated by the monomer droplets. These results demonstrate that shorter polymer chains (DP 50) allow for sufficiently PMMAcompatible CNCs but with less tendency to entangle or form a separate CNC rich phase, and therefore, the DP 50 polymergrafted CNCs were small enough to be incorporated inside the polymer latex particles. In addition to the work presented above, PMMA latexes containing PMMA-grafted CNCs (not PBA-grafted) were prepared. CNC-g-PMMA200 was added to the MMA monomer phase and the miniemulsion polymerization was carried out according to the same protocol as for CNC-g-PBA50. However, when the latexes were imaged by TEM, CNCs were not observed inside the latex (Supporting Information, Figure S4), and rod-shaped CNCs were found separate from the polymer particles in the filtered latex as in the case of CNC-g-PBA200 (Supporting Information, Figure S3b). As such, the grafted polymer type did not have an observable effect on the incorporation of polymer-grafted CNCs, since no difference between CNC-g-PBA200 and CNC-g-PMMA200 was seen by TEM or other physical characterization methods (Supporting Information, Figure S3 and Table S2). Unmodified CNCs have contact angles between 13 and 25°,54 while the polymer-grafted CNCs described here had increased contact angles of 50−130° (Supporting Information, Table S1), implying a decrease in hydrophilicity and an overall increase in the compatibility with hydrophobic MMA/PMMA. Specifically, CNC-g-PBA50 had a contact angle of 50°, while CNC-g-PBA200 and CNC-g-PMMA200 had contact angles of 130° and 120°, respectively. Since PMMA and PBA polymers have similar hydrophobicities, PBA-grafted CNCs were compatible with PMMA and we expect the converse to be true as well. Furthermore, we expect that PBA/PMMA-grafted CNCs can both be incorporated into PBA latexes, which are common in latex-based pressure sensitive adhesives but were not investigated here as PBA is a harder system to image due to the low Tg of PBA and flattening/coalescence of the polymer particles. As such, we are merely claiming that PMMA is a good model latex system to observe CNCs inside the particles and believe that the findings are applicable to many latex systems. The contact angle results imply that a balance is required between increasing hydrophobicity and avoiding

Figure 3. TEM micrographs of PMMA latexes containing (a) no CNCs, (b) unmodified CNCs, (c) CNC-g-PBA50 and inset of other latex particles with CNC-g-PBA50, and (d) CNC-g-PBA200. Scale bars are 400 nm.

rod-shaped CNCs outside of the particles, shown in Figure 3b. However, in the case of the latexes with CNC-g-PBA50, larger polymer particles (400−600 nm) were observed with rodshaped polymer-grafted CNCs inside the spherical latex particles, and no CNC aggregates were found outside the latex particles (Figure 3c, with insets showing other latex particles imaged from the same sample). As TEM is a 2D image of dried spherical 3D particles, the technique makes it difficult to determine whether the CNC-g-PBA50 are dispersed within the bulk of the polymer particle, or simply inside the PMMA/water interface. However, TEM still provides an accurate and reliable indication of whether the polymer-grafted CNCs are inside or outside the PMMA latex particles. Furthermore, scanning electron microscopy images of the PMMA latex containing CNC-g-PBA50 showed that the polymer-grafted CNCs were not at the interface (Supporting Information, Figure S2) and were therefore not acting as Pickering stabilizers and must be incorporated inside the PMMA latex particles. When CNC-g-PBA200 was added to the monomer phase, TEM showed an increase in latex particle size and degree of aggregation, but did not show polymer-grafted CNCs inside the particles (Figure 3d). We note that drying the polymer particles on the TEM grids for imaging leads to a certain degree of flattening (as seen significantly for low Tg adhesive latexes14) such that particles appear larger and cannot be directly compared to light scattering sizes, which are measured in water-based dispersion. It has been reported that highly crystalline particles cause a distinct diffraction of the electron beam in TEM,53 and interestingly, when the CNCs were uniformly dispersed throughout the latex suspension, such diffraction was undetectable; however, the diffraction of the electron beam (observed as a “halo” around the latex particles when outside of their focal plane, due to the presence of crystalline CNCs) was observed when the CNCs were clustered within the 993

DOI: 10.1021/acsmacrolett.8b00334 ACS Macro Lett. 2018, 7, 990−996


ACS Macro Letters Present Address

CNC entanglement/aggregation to incorporate CNCs into latex particles. Additionally, these contact angles suggest that other surface modifications such as grafting smaller hydrophobic molecules, other hydrophobic polymers with short chain lengths, or surfactant adsorption may be sufficient, provided they do not interfere with the polymerization mechanism (e.g., no radical scavengers) or dispersion stability. While the sample preparation and the drying of latexes for TEM imaging may change the appearance and degree of aggregation of the samples, we believe that the combination of DLS and zeta potential results with TEM give us a good representation of these new nanocomposite latex systems. Overall, they are colloidally stable, well dispersed, and mostly in the nanoscale, and we have strong indication that the CNCg-PBA50 are inside the PMMA latex particles. This is the first report showing encapsulation of CNCs inside polymer particles. The latex nanocomposite prepared here with CNC-g-PBA50 is anticipated to have better mechanical properties compared to PMMA latexes without reinforcing agents, and compared to PMMA-CNC nanocomposites compounded more conventionally (i.e., by melt-mixing, ball milling,55 or in situ bulk polymerization56), due to the incorporation and uniform distribution of CNCs. For such performance tests, scale up of the current syntheses is required and is currently underway. Recently we showed that in pressure-sensitive adhesive latexes with monomers that are more hydrophobic than BA/MMA, severe aggregation of CNCs in the water phase leads to decreased material performance. For such latex systems, compatibilization of CNCs for incorporation into the latex particles would be ideal. We believe the proof-of-concept work demonstrated here has significant industrial implications, where various CNC modification routes are possible to improve compatibility to control the location and distribution of CNCs in latex composites for coating, adhesive and specialty applications.

Department of Chemical and Petroleum Engineering, University of Calgary, 2500 University Drive NW, Calgary, AB, T2N 1N4, Canada.


The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors are grateful to CelluForce Inc. for providing the unmodified CNC starting material, and to Prof. Pelton and Prof. Adronov for equipment use. The Biointerfaces Institute at McMaster is acknowledged for equipment use and training. Special thanks to Marcia Reid and the Electron Microscopy Facility in the Health Science Center at McMaster University, and Daniel Osorio for obtaining SEM images at the Canadian Center for Electron Microscopy at McMaster University. CelluForce Inc., FPInnovations, and the Natural Sciences and Engineering Research Council of Canada (NSERC) are acknowledged for financial support and funding through NSERC Grant CRDPJ492852-15.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00334.


Experimental materials and methods, characterization of free polymer, contact angle of polymer-grafted CNC films, ATR-FTIR spectra of unmodified, BiB-, and polymer grafted-CNCs, SEM micrograph of PMMA latex containing CNC-g-PBA50, TEM micrographs of filtered PMMA latexes containing CNC-g-PBA200 and CNC-g-PMMA200, TEM micrograph of PMMA latex containing CNC-g-PMMA200, and characterization of physical properties of PMMA latex containing CNC-gPBA200 and CNC-g-PMMA200 (PDF).


Corresponding Author

*E-mail: [email protected]. ORCID

Stephanie A. Kedzior: 0000-0003-3112-3890 Emily D. Cranston: 0000-0003-4210-9787 994

DOI: 10.1021/acsmacrolett.8b00334 ACS Macro Lett. 2018, 7, 990−996


ACS Macro Letters

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DOI: 10.1021/acsmacrolett.8b00334 ACS Macro Lett. 2018, 7, 990−996