820 Macromol. Rapid Commun. 21, 820–824 (2000)

Communication: Stable dispersions of polyacrylonitrile (PAN) nanoparticles in the size range between 100 nm a d a 180 nm were made by polymerization in miniemulsion and characterized by dynamic light scattering, transmis- sion electron microscopy (TEM), and wide angle X-ray scattering (WAXS). Due to the insolubility of the in its monomer, such particles are not accessible by classi- cal emulsion polymerization. The pure PAN particles are composed of ca. 10 nm large polymer nanocrystals, i.e., the formed polymer precipitates and crystallizes direct after formation. As a consequence, the final latexes do not adopt spherical shape, but show a well defined, narrowly distributed boulder-like phenotype which is called “crumpled latexes”. Copolymerization with results in a continuous transition between the crumpled and a smooth spherical morphology, which is again related to a decreased degree of crystallinity.

Transmission electron microscope photograph for a pure PAN latex showing a crumpled appearance

The polymerization of in miniemulsions: “Crumpled latex particles” or polymer nanocrystals

Katharina Landfester*, Markus Antonietti Max Planck Institute of Colloids and Interfaces, Research Campus Golm, D-14424 Potsdam, Germany (Received: April 3, 2000; revised: May 15, 2000)

Introduction Due to the strong molecular interactions, PAN is not Polyacrylonitrile is an interesting starting polymer for a soluble in the monomer (a peculiarity it shares with vinyl whole range of engineering applications1). Although pro- chloride and which is not found for styrene and all other duced by radical polymerization process as a mainly atac- common monomers). This makes it very difficult to tic polymer, polyacrylonitrile is known to form crystal- homopolymerize acrylonitrile in an emulsion polymeriza- lites by virtue of the mutual segregation of crystallizable tion process since polymer particles, once nucleated, can- sequences2). The degree of crystallinity in polyacryloni- not grow by diffusion of monomer into the nucleated trile (PAN) is substantially greater than that predicted polymer particles to form intermediate monomer swollen from the tacticity. Crystallinity degrees of 28 to 34% for particles, the only species which effectively promotes atactic PAN (about 40% syndiotactic dyads) are reported polymerization in this two phase situation. Therefore, in in the literature3). The strong mutual interaction between the literature only the formation of particles, the polymer chains and the crystalline nature is the base e.g., with styrene, acrylates or butadiene and styrene, is for the use of polyacrylonitrile in fibers and as a compo- described. nent in engineering plastics, e.g., in housing and packag- In microemulsions, the copolymerization behavior of ing applications. Carbonization of PAN fibers and films acrylonitrile and styrene was studied for compositions results in fibers and membranes with extraordin- with acrylonitrile contents from 10 to 90 wt.-%, but only ary properties where again use was made of the very spe- at low conversions4). Stable microemulsions are obtained cial chemistry and interactions of this monomer. for 25/75 polyacrylonitrile/polystyrene (PAN/PS) mix-

Macromol. Rapid Commun. 21, No. 12 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2000 1022-1336/2000/1208–0820$17.50+.50/0 The polymerization of acrylonitrile in miniemulsions ... 821 tures5). Copolymerizations of acrylonitrile with varying where the resulting polymer dispersion are nanocrystal- comonomers are also discussed in case of emulsion poly- line. The influence of a potential comonomer styrene on merization. The influence of main operating variables particle morphology and the crystallinity will be also dis- (such as the number of seed polymer particles, the como- cussed in the paper. nomer composition, and the initiator concentration) on the rate of copolymerization, the composition and other properties of the resultant copolymer was stud- Experimental part 6, 7) ied in many papers . Saturation swelling studied of styr- Synthesis of the latexes ene-acrylonitrile copolymer particles with styrene and 6 g of the monomer (acrylonitrile or acrylonitrile/styrene acrylonitrile monomers showed that with increasing acry- mixtures), 250 mg of hexadecane, and different amounts of lonitrile content, the swellability of the copolymer parti- the initiator V59 (2,29-azo(2-methylbutyronitrile)) were 8) cles decreases as expected . mixed and added to a solution of 120 mg sodium dodecyl Another experimental approach to generate at least sulfate (SDS) in 24 g water (if different amounts of SDS are partly pure PAN nanostructures is to use a polymer seed used it is indicated in the text). After stirring for 1 h, the of different chemical composition. Dimonie et al. miniemulsion was prepared by ultrasonicating the emulsion reported of the core-shell emulsion copolymerization of for 120 s at 90% amplitude with a Branson sonifier W450 styrene and acrylonitrile on polystyrene seed particles9) or Digital. To avoid any polymerization due to a heating up of on poly(butadiene-co-styrene) particles10). In this case, the sample, the mixture was ice-cooled during the homogeni- the acrylonitrile chains are precipitated in the presence of zation. For polymerization, the temperature was increased to 8 the comonomer onto already existing polymer core parti- 72 C. After 4 h, full conversion of the monomers was obtained. For pure PAN latexes, the polymerization tempera- cles which are monomer-swollen and serve as the source ture was chosen to be 558C in order to better control the of monomer. Also by precipitation of acrylonitrile on a polymerization. polystyrene core, but without the use of an emulsifier, core shell particles with a polystyrene core and a poly- acrylonitrile shell were prepared. These particles were Analytical methods pyrolyzed in order to generate nanoscopic fine hollow The particle size analysis was conducted by using a Nicomp carbon particles11). A copolymer layer of styrene and particle sizer (Model 370, PSS Santa Barbara, USA) at a 8 acrylonitrile can also be synthesized in an emulsion poly- fixed scattering angle of 90 . merization process onto montmorillonite sheets12). For the transmission electron microscopy (TEM) micro- graphs the samples were diluted and mounted on copper Sahoo et al. described the emulsifier-free emulsion 13) grids, which were coated with carbon. These grids where polymerization of acrylonitrile , but during the course of then examined in an EM 902 (Zeiss, Germany). the polymerization, the polymer precipitated. Concen- The wide angle diffractograms were measured with a wide trated dispersion polymerization of PAN in which the angle goniometer (Enraf-Nonius PS-120, Cu Ka) with a lin- volume fraction of the dispersed phase is between 0.74 ear local sensitive detector. The samples were measured in and 0.99 was performed by Ruckenstein and Li14). reflexion/transmission. With the suspension polymerization process one can in principle overcome the problem of the insolubility of PAN in AN since diffusional transport of monomer to the Results and discussion locus of reaction is not a rate determining step here. How- Miniemulsions of acrylonitrile can easily be formulated ever even for the suspension polymerization it is mostly with hexadecane as a hydrophobe and sodium dodecyl the formation of copolymer particles which is described sulfate (SDS) as a surfactant. After polymerization of the in the literature15, 16). droplets, depending on the surfactant amount, PAN- Recently, our group developed on the base of former homopolymer particles with a diameter between 100 and developments17–20) a quantitative understanding of the 180 nm and a polydispersity smaller than 10% Gaussian miniemulsion process21–23). Here, small monomer droplets width were obtained. It has to be noted that the more SDS in the range of 30 to 300 nm are formed by a high energy is used and therefore the smaller the droplets are, the dispersion process, and in the ideal case, these droplets more difficult it is to control the polymerization of the are copied in a 1:1 process to the final polymer latexes. droplets. The analytical data of the PAN latexes are sum- In miniemulsions, diffusion of monomer is not relevant, marized in Tab. 1. since the monomer is already from the beginning close to As compared to the standard styrene recipes21–23), it has the locus of reaction. This makes the miniemulsion very to be considered that the solubility of acrylonitrile in promising for the synthesis of stable PAN latex particles. water is rather high (7.35 wt.-%). For a miniemulsion It will be shown below that miniemulsion polymeriza- with 20 wt.-% acrylonitrile, just about 70% of the mono- tion is indeed a direct and convenient way to generate mer is therefore located in the droplets, and 30 wt.-% is pure PAN particles of various sizes and morphology dissolved in the water phase. This is however no restric- 822 K. Landfester, M. Antonietti

Tab. 1. Analytical data of the PAN latexes

Latex SDS amount diameter ————— ——— wt.-% nm

PAN-1 2.0 180 PAN-2 4.0 117 PAN-3 6.0 98

tion for the miniemulsion polymerization process, and the use of an oil-soluble initiator (V59) allows preservation of each droplet as the nanosized location of reaction. It is just that slight particle growth by polymer precipitation onto the particles has to be expected. Initiation in the water phase together with the high water solubility of the monomer would result in polymerization in the water phase and secondary nucleation of new particles, i.e., loss of reaction control and the size control. The absence of free or dissolved monomer in the polymerized latexes was confirmed by NMR. During the polymerization, each polymer chain crystal- lizes and precipitates within the minidroplets, and the resulting structure is more an aggregate of simple poly- mer nanocrystals than a traditional latex particle. This is why the PAN particles are uniform in overall size, but not smooth spherical at all. The TEM pictures (Fig. 1) show a boulder-like or crumpled appearance where the single polymer nanocrystals remain in the final structure and are easily identified by their sharp edges and flat surfaces. The crystallinity of the particles was measured by wide angle X-ray scattering (WAXS) measurements shown in Fig. 2. For the pure PAN latexes, an apparent degree of crystallinity of 26% was obtained by integration of the WAXS peaks, which is rather high for an atactic polymer. In a next set of experiments, acrylonitrile was copoly- merized with increasing amounts of styrene (Tab. 1). For an otherwise constant set of reaction parameters, the par- ticle size decreases with increasing styrene content. An addition of 10 wt.-% styrene decreases the particle size

Tab. 2. Characteristics of the PAN latexes with different amounts of PS, the SDS amount was 2 wt.-%

Latex Diameter Crystallinity ———— ———— nm %

PAN/PS 100/0 180 26 PAN/PS 90/10 163 17 PAN/PS 80/20 117 11 PAN/PS 70/30 112 8 Fig. 1. Transmission electron micrographs of sample PAN-1 PAN/PS 60/40 117 6 PAN/PS 50/50 84 3 PAN/PS 40/60 80 0 from 180 to 163 nm, between 20 and 40 wt.-% of styrene PAN/PS 30/70 72 0 PAN/PS 20/80 78 0 the particle size is about 115 nm, and at contents higher PAN/PS 10/90 76 0 than 40 wt.-% the particle size decreases to ca. 80 nm. PAN/PS 0/100 84 0 This is interpreted by the fact that SDS is obviously not The polymerization of acrylonitrile in miniemulsions ... 823

(a)

Fig. 2. WAXS data for PAN latexes with different amount of PS

(b) an optimal surfactant for the very hydrophilic acryloni- trile, i.e., the binding between the C12 tail and the AN is weak. This also points towards a possible improvement for the efficiency of this reaction by appropriate choice of a polar surface stabilizer. The degree of crystallinity depends on the amount of styrene (Fig. 2). With increasing amount of styrene, the degree of crystallinity decreases. At styrene content of higher than 50 wt.-%, the WAXS-crystallinity has disap- peared. TEM photographs also illustrate that with increasing PS amount, the particles are less crumbled (Fig. 3). At 50 wt.-% PS, the particles are smooth. The crystallite sizes can be estimated from the line width of the WAXS peaks using the Scherrer equation. Independent of the amount of styrene (0 to 50 wt.-%), crystallite sizes of about 12 nm were detected. This (c) agrees well with the size of the structures seen in the elec- tron micrographs for the pure PAN particles. From the reactivity parameters it is well known that the formation of a alternating copolymer is preferred (values of rAN = 24) 0.02 to 0.17 and rSt = 0.14 to 0.55 are reported . The DSC measurements do not show a for pure PS indicating the formation of a copolymer.

Conclusion Polymerization in miniemulsion is a very suitable techni- que to obtain pure polyacrylonitrile (PAN) nanoparticles in the size range between 100 nm a d a 180 nm. Since the polymer is insoluble in the monomer, the formed polymer precipitates during the polymerization within the droplets, and ca. 10 nm large polymer nanocrystals are Fig. 3. TEM photographs for PAN latexes with different formed. Pure PAN latexes have a crumpled appearance. amount of PS a) 100 wt.-% PAN; b) 80 wt.-% PAN; c) 50 wt.-% PAN A continuous transition between the crumpled and a smooth spherical morphology can be obtained by copoly- merizing acrylonitrile with increasing amounts of styrene. where the polymer is not soluble in the monomer. It also The results are related to a decreased degree of crystalli- exemplifies that crystallization processes in nanoparticles nity. The work shows that the miniemulsion can be used are possible, giving access to polymer crystals as a new in an excellent way for the preparation of latex particles type of polymer filler and modifier. 824 K. Landfester, M. Antonietti

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