Rather than modifying the clay with organic quaternized ammonium salts, cationically modified polymerization initiators can also be used to prepare organophilic clays. In this method, in the situ polymerization is initiated by the activation of these initiators which are ionically bound to the clay particle surfaces, that is, through a surface-initiated polymerization (SIP) process. The advantage of SIP is based on the assumption that as the polymer chain grows through surface initiation, the ordered silicate layers can be gradually pushed apart, ultimately exfoliating to discrete laths, resulting in a well-dispersed structure of the final product. Also, theoretically, if all initiators are tethered to clay surfaces, a higher efficiency of intergallery polymerization is expected compared to that of free, or unattached initiators. Exfoliated polystyrene-clay nanocomposites with controllable MW have been prepared by intercalating a charged living free radical polymerization (LFRP) initiator into montmorillonite. [33] A (1,1-diphenylethylene) DPE derivative initiator was used to synthesize polystyrene-clay nanocomposite materials through living anionic surface-initiated polymerization (LASIP). [34,35] However, only intercalated structures were obtained.
In efforts to conduct SIP from clay surfaces, Xiaowu and co-workers [36]
recently synthesized two initiators for free radical SIP, both contain quaternized amine endgroups for cation exchange with montmorillonite particles. The initiator molecule design is as follow: (1) symmetric, with two cationic groups at both chain ends (named bicationic free radical initiator hereafter) and (2) asymmetric, with one cationic group at one end (named monocationic free radical initiator hereafter). The synthetic schemes and structures of these initiators are shown in Figures 1-12a and 12b. They are both AIBN-analogue initiators for free radical polymerization. The use
of another symmetric bicationic azo compound, 2,2’-azobis(isobutyramidine hydrochloride) (AIBN), has also been proven to be feasible for styrene SIP on high surface area mica powder. [37] However, no structural information for these SIP products has been reported. Asymmetric azo initiators in the form of silanes have also been successfully employed to free radically polymerize styrene from spherical silica gel surfaces. [38,39] To the best of our knowledge, there have been no reports on a direct free radical SIP approach from surface-bound monocationic azo initiators on individual clay nanoparticles.
-Figure 1-12 (a) Synthetic scheme and structure of the bicationic free radical initiator.
(b) Synthetic Scheme and structure of the monocationic free radical initiator. [36]
X-ray powder diffraction patterns of the pristine clay and two initiator-intercalated clay samples are shown in Figure 13. Lamellar periodicity was maintained on the organophilic clay despite the rigorous sonication-centrifugation procedure to intercalate the initiators. By using the Bragg equation, nλ=2dsinθ, the d spacing values of these samples were calculated and shown beside each peak.
accordance with data from other sources. [40] The XRD patterns of the intercalated clays indicate the successful insertion of the initiator molecules into the galleries of the silicate platelets since both intercalated clay samples gave increased d spacing values. In addition, the sharper shape and the higher diffraction intensities of these peaks after intercalation provide the evidence of a better-ordered swollen structure than that of the original clay. This result demonstrates that the layered framework of inorganic clay can accommodate the AIBN derivative molecules of various functionalities with better long-range periodicity.
Figure 1-13. X-ray powder diffraction patterns of pure clay and two intercalated clay samples. [36]
On further analysis, the d spacing values seemed to be inconsistent with the steric sizes of the two initiators. The d spacing of bicationic intercalated clay (1.52 nm) is substantially smaller than that of the monocationic intercalated clay although their molecular dimensions are comparable (both chain length values are 2.20 nm, as estimated by Chem 3D software). The bicationic initiator molecule possesses charged groups on both ends that can have two intercalation possibilities: (1) these two cationic endgroups interact electrostatically with two different but neighboring
platelets’ surfaces, or (2) they interact on the same side surface of a single clay particle. The combination of these two possibilities makes the intercalated structure less spatially ordered which accounts for the broadened reflection for this sample as compared with the peak of the clay intercalated by the monocationic initiator.
Furthermore, since XRD collects the average information from a large area of a powder sample, a synergic effect of these two possibilities accounts for an intermediate d spacing value. This interpretation is schematically shown in Figure 1-14. The interlamellar height shown in the figure is calculated by Δd = d spacing – thickness of one platelet (~1.0 nm).
Figure 1-14. (a-c) Schematic diagrams of the intercalation: (a) original clay, (b) clay intercalated with bicationic initiator, and (c) clay intercalated with monocationic
initiator. [36]
The X-ray diffractograms of the two final SIP products are shown in Figure 1-15.
to the long-range order of the polystyrene matrix. Similar broad peaks were also observed in the diffractogram of the PS-0 reference sample (not shown). Sample bi-PS-M-2 shows a small peak at 2θ=5.9o, which is about the same as the peak position of the corresponding intercalated clay (Figure 1-8), implying that this product still contains fraction of the intercalated clay structure. On the contrary, there is no peak on the XRD trace of the mono-PS-M-2 sample, indicating that a completely exfoliated structure was achieved in this sample.
Figure 1-15. XRD spectra of the two SIP nanocomposites showing degree of exfoliation. [36]
This observed result is quite unexpected. We would anticipated that these adjacent clay layers in the clay/bicationic initiator system will be gradually pushed apart during SIP, if these two immobilized free radicals are simultaneously generated.
As a result, the intercalated clay stacks would be totally delaminated, forming a fully exfoliated nanocomposite product. However, the polymer can only grow within the clay gallery when monomer molecules are able to diffuse and make contact with effectively with the tethered radicals within the interlayer spacing. The time scale of diffusion is such that access to the monomer from within the layers is limited.
Considering the rapid kinetics for free radical polymerization in solution, this intercalative monomer diffusion is significantly slower toward monomer addition.
Thus, free initiators exhaust the monomers while SIP inside clay lamellar is delayed by diffusion. Furthermore, there is also competition from the surface-perimeter-attached initiators of the clay particles. Even if some of the bication initiators were activated and grew to become oligomers, the growing chains will likely be terminated by recombination or disproportionation by nearby immobilized growing chains/initiators in the same gallery. Hence the low molecular weight and high polydispersity obtained by bi-PS-M-2 can be explained.
By comparison, an intercalated monocationic initiator is easier to be delaminated than a bicationic initiator. The monocationic initiator molecule is also more organophilic. The weaker van der Waals interaction between the alkyl headgroups of the monocationic initiator and clay surfaces makes the intercalated clay easier to be swelled by the solvent and monomer. Once the clay intercalated with monocationic initiator is exfoliated by sonicating and stirring, the attached initiators have more accessibility to monomer and thus results in better monomer intercalative diffusion.
1.5 Types of Polymer matrix