Polyolefin/POSS and norbornyl/POSS copolymers

4.1.1. Polyethylene and norbornyl/POSS copolymers A number of interesting design strategies for the prepa-ration of polyolefin/POSS hybrid materials have evolved

over the past decade [42–50]. Coughlin and coworkers [42] synthesized PE hybrid containing POSS through ring-opening metathesis copolymerization (Fig. 12). From studies of nanostructured PE-POSS copolymers through controlled crystallization and aggregation, they found that two distinctly different crystallizing components were present in these copolymers and the final structure depended on the respective crystallization kinetics under different crystallization conditions[42].Fig. 13 presents TEM micrographs of polybutdaine-POSS (PBD-POSS) ran-dom copolymers with POSS contents of 12 and 43 wt%[51].

InFig. 13(a), POSS aggregates are clearly observed as short randomly oriented lamellae having lateral dimensions of ca. 50 nm. The thickness of the lamellae ca. 3–5 nm, roughly corresponds to twice the diameter of a POSS NP. Increasing the incorporation ratio of POSS to 43 wt% resulted in the formation of continuous lamellae having lateral lengths on the order of microns (Fig. 13(c)). The irregular lamellar spacing observed in this image possibly arose from a combination of both twisting of the POSS lamella and the random nature of the copolymers. The morphology bears similarity to the lamellar morphology formed by precise diblock copolymers [51]. Furthermore, Coughlin and coworkers [42,52] and Mather et al. [53] reported polyolefin copolymers containing norbornyl–POSS macromonomers. Polyolefin-POSS copolymers incor-porating norbornylene–POSS macromonomer have been prepared using a metallocene/methyl aluminoxane (MAO) co-catalyst system[52]. Using a Pd-diimine catalyst, Ye and

Fig. 11. Polymer/POSS architectures.

coworkers[54]synthesized hyperbranched PE containing covalently tethered POSS NPs through chain-walking ethylene copolymerization with a POSS macromonomer bearing a polar acryloisobutyl-POSS unit. The covalent incorporation of the high-mass POSS NPs significantly reduced the intrinsic viscosity of the copolymers relative

to the pure PE of the same molecular weight, owing to the highly compact spherical cage structure of the POSS NPs. Thermal studies confirm that the incorporation of POSS units significantly enhanced the thermal oxidative stability of the polymers in air, with the value of Tgof the copolymer increasing upon increasing the POSS contents.

Fig. 12. Copolymerization of cyclooctene and norbornene–POSS.

Fig. 13. (a) TEM of polybutaidiene-POSS (PBD-POSS) copolymer with 12 wt% of Cp-POSS. (b) Schematic drawing of PBD-POSS assembly at low POSS concentration. (c) TEM of PBD-POSS copolymer with 43 wt% of Cp-POSS. (d) Schematic drawing of PBD-POSS assembly at high POSS concentration.

Reprinted with permission from Ref.[51]. Copyright 2001, American Chemical Society, USA.

Joshi et al.[47]used a melt mixture route to prepare high-density PE/octamethyl-POSS (OM-POSS) nanocom-posites. The rheological results revealed that, at lower filler contents (0.25–0.5 wt%) the POSS particles acted as a lubricant to reduce the complex viscosity of the nanocom-posites. At higher POSS concentrations, the viscosities of the nanocomposites increased. The POSS derivatives remained miscible with HDPE at lower concentrations and lower temperatures, but they tended to aggregate at higher concentrations and higher temperatures. From studies of non-isothermal crystallization of the HDPE/OM-POSS nanocomposites using the Kissinger method [49], they also found that the presence of the POSS units did not have any significant effect on the activation energy for the transport of the polymer segments to the growing surface.

They observed that only those POSS units dispersed at the molecular level could act as nucleating agents while the POSS nanocrystals did not affect the crystallization process.

4.1.2. Polypropylene/POSS nanocomposites

Zhang et al. [55] prepared polypropylene-POSS nanocomposites using a C2 symmetric ansa-metallocene catalyst in conjunction with a modified MAO. Their PP/POSS copolymers exhibited improved thermal

sta-bilities with higher degradation temperature and char yields, revealing that inclusion of the inorganic POSS NPs made the organic polymer matrix more thermally robust. Hsiao and coworkers[56]used DSC to investigate a series of isotactic polypropylene (iPP) melt-blended with nanostructured OM-POSS molecules to study the quiescent melt crystallization behavior and shear-induced crystallization behavior. They observed that the addition of OM-POSS molecules increased the crystallization rate of iPP under both isothermal and non-isothermal conditions, implying that POSS crystals acted as nucleating agents, a finding that is similar to that with PE systems. Tabuani and coworkers[57–61]reported the influence of the POSS substituent on the morphological and thermal character-istics of melt-blended PP/POSS composites [57–61]. By varying the amount of filler, they investigated the effects of three different alkyl substituents (R = ethyl, isobutyl, iso-octyl) in the POSS structure on the morphological characteristics, crystallization and melting behavior of PP/POSS composites. They found that the lengths of the alkyl groups of the POSS molecules played a fundamental role affecting the degrees of dispersion and interactions with the PP matrix during the cooling process from the melt.

Fig. 14. Reaction between PP-g-MA and NH2-POSS producing imide bond.

Two metal-POSS (M-POSS) systems, Ti(IV)- and Al(III)-isobutyl-POSS were blended with PP to study the morphological, crystallization, and thermal behavior of these M-POSS/PP composites[61]. Ti- and Al-POSS had different effects on the thermoxidative behavior of the polypropylene matrix in the M-POSS/PP composites, revealing the clear specificity of the metal center on the PP degradation pathway. In particular, Ti-POSS significantly stabilized PP when heated in air, whereas Al-POSS had only limited effects. Moreover, Ti-POSS affected the crystalliza-tion of PP, driving the crystallizacrystalliza-tion process along specific crystallographic directions.

Fina et al.[62]reported the maleic anhydride-grafted polypropylene (PP-g-MA)/POSS hybrids prepared through grafting of aminoethylaminopropyl heptaisobutyl POSS (AM-POSS) in a one-step reactive blending process (Fig. 14).

Morphological analyses revealed the dispersion of POSS units on the nanoscale because of the high chemical reac-tivity between POSS and PP-gMA. The presence of grafted POSS moieties improved the thermoxidative stability of PP-g-MA, in terms of delaying mass loss during thermal degradation under air, relative to pure g-MA and PP-g-MA containing comparable amount of the non-reactive OiBu-POSS.

Chen and coworkers [63,64] studied the isothermal crystallization kinetics and morphological development of iPP blended with small loadings of OM-POSS. The pre-dominantly nanocrystalline POSS acted as an effective nucleating agent to promote the nucleation rate of iPP.

In contrast, the minor amount of those slightly misci-ble and dispersed POSS molecules retarded the nucleation and growth rates of iPP in the remaining bulk region.

Zhou et al.[65–68]studied the crystallization behavior of PP/octavinyl-POSS (OV-POSS) prepared using two different processing methods: reactive blending and physical blend-ing. Crystallization in the PP/POSS composites was strongly influenced by the processing method. POSS particles can act as effective nucleating agents to accelerate the crys-tallization of PP. The cryscrys-tallization rate increased more dramatically for the reactive blending composite because of the stronger nucleating effect of PP-grafted POSS. The surface energy of chain folding of the physical-blend and

reactive-blend composites changed from 156.5 mJ/m²for pure PP to 81.2 and 24.5 mJ/m², respectively[62].

Misra et al.[69]used melt blending to study the sur-face energy and mechanical behavior of PP/OiBu-POSS nanocomposites. Incorporation of 10% POSS resulted in a 3% reduction in the surface energy (to 24 mN/m), and a 27%

increase in the water contact angle (to 99^◦). The increased water contact angle reveals the hydrophobic nature of the PP/POSS nanocomposite surface. The observed increases in surface hydrophobicity by increasing POSS concentration can be related to surface roughness through AFM roughness analysis[69]. Tang and Lewin[70]studied the migration and surface modification of PP/POSS nanocomposites by annealing the melt and by heating the solid blend in a microwave oven. Their static contact angle measurements revealed very high hydrophobicity as well as low surface free energy of the surface of the annealed sample, close to Teflon or pristine POSS. The migration of POSS was due to its lower surface tension and lower cohesive energy with the matrix chains relative to the cohesion energy between polymer chains, and the density and temperature fluctu-ations of the matrix chains upon relaxation repulse. As a result, the POSS units were propelled to the surfaces[70].

4.1.3. Other polyolefin POSS nanocomposites

Cohen and coworkers[71,72]found that octamethacryl-POSS (OMA-octamethacryl-POSS) has the ability to plasticize PVC better than organic plasticizers such as dioctyl phthalate (DOP).

They found that the value of Tg of ternary blends of PVC/POSS/DOP could be reduced to near room temper-ature. Zheng et al. [73] reported that the inclusions of octaglycidyl dimethylsilyl POSS (OG-POSS) into PEI resulted in higher values of Tg and enhanced thermal stabilities relative to those of pure PEI. In addition, PEI containing hepta(3,3,3-trifluoropropyl) glycidylether-propyl POSS exhibited typical amphiphilicity as evidenced by increased surface hydrophobilicity.