Mechanical performance of hybrid nanocomposites obtained by reactive blending

3 May 2013
Zbigniew Bartczak and Magdalena Grala
Modification of polypropylene by grafting with polyhedral oligomeric silsesquioxanes cage molecules enhances oxidation resistance, and does not markedly affect its mechanical properties except for ultimate elongation.

Polyhedral oligomeric silsesquioxanes (POSS) exhibiting a specific cage structure have attracted increasing attention as components of nanomaterials due to their truly hybrid inorganic–organic architecture. This consists of an inner, robust silicon-oxygen framework (SiO1 .5 )x, externally covered by organic substituents that can feature a range of polar or nonpolar functional groups.1, 2 Due to the variety of organic substituents, POSS derivatives can be incorporated effectively into polymers by copolymerization, grafting, or even blending. Blending techniques seem the most attractive because they are simple and inexpensive.

Fine dispersion of POSS is a crucial issue in blending since full advantage of POSS properties can only be taken when it is dispersed at the molecular level or in nanosized clusters. However, the poor interactions between POSS and most organic polymer matrices, as well as strong self-interaction between geometrically regular POSS cages, frequently results in POSS aggregation.3

In this work we focused on the preparation and mechanical performance testing of hybrid nanocomposites obtained by reactive melt blending of isotactic polypropylene (iPP) and polypropylene (PP) grafted with maleic anhydride (PP-g-MA) with small amounts of POSS functionalized with amine groups4 (see Figure 1 for the reaction scheme). The objective was to obtain material with POSS dispersed at the molecular level. Considering POSS molecules as the smallest particles of silica (diameter ~1nm), one could expect to obtain a nanocomposite with extremely small and finely dispersed nanofiller particles. The robust inorganic core of POSS might then act as molecular reinforcement, improving properties of the polymeric matrix even at low POSS concentrations. The interesting question was whether nanoparticles as small as single POSS molecules could really reinforce a matrix in a similar way to some larger particles, and what the mechanism for that reinforcement might be.


Melt grafting reaction of amine-functionalized polyhedral oligomeric silsesquioxane (POSS) onto polypropylene (PP) grafted with maleic anhydride (PP-g-MA) leading to the formation of PP-g-POSS hybrid nanocomposite.

We prepared polypropylene-POSS grafted (PP-g-POSS) hybrid nanocomposites by reactive mixing of PP-g-MA with 5 or 10% by weight of amine-functionalized POSS (the molar ratio of MA to NH2 groups was approximately 2:1 or 1:1, respectively) in a Brabender internal mixer. In the next processing step, the PP-g-POSS hybrids were diluted with iPP homopolymer in a 1:1 or 1:3 weight ratio by blending in a single screw extruder. This allowed us to obtain iPP-based nanocomposites with low concentrations of finely dispersed POSS, varying from 1.25 to 5% by weight. For reference purposes, we prepared blends of similar PP-POSS overall compositions but with all PP-g-MA replaced by iPP homopolymer (i.e., iPP/POSS physical binary blends) as well as the binary blends of iPP with PP-g-MA (without POSS) using the same protocol.4 We verified grafting of POSS in the prepared nanocomposites by FTIR, which revealed that the imide band5 (1703cm1) developed during reactive blending. This confirmed the reaction of maleic anhydride with amine, and therefore that POSS molecules were grafted onto the PP backbone.4 We also examined the phase structure using scanning electron microscopy (SEM) and x-ray diffraction. SEM observations revealed small POSS crystals in the iPP/POSS physical blends, and an absence of any crystals or other aggregates in the nanocomposites obtained from reactive blending (see Figure 2). This quasi-homogeneous phase structure was corroborated using x-ray diffraction.4 Differential scanning calorimetry and small angle x-ray scattering demonstrated lower crystallinity, melting temperature, and long period (thus indicating reduced lamellae thickness) of nanocomposites compared to iPP.4


Scanning electron micrographs of freeze-fracture surfaces of: (a) iPP/POSS physical blend crystallized isothermally at 132°C, and (b) PP-g-POSS obtained by reactive blending. Both samples contain 5% by weight of aminopropylheptaisobutyl-POSS.

Thermal stability studies revealed that iPP, PP-g-MA and PP-g-POSS heated in an inert atmosphere behave in a similar fashion. This is typical for thermal degradation of polypropylene. Heating in an oxidative atmosphere results in accelerated degradation due to peroxidation.6 However, PP grafted with POSS demonstrated a significantly higher decomposition temperature than iPP or PP-g-MA—see Table 1. The improved thermo-oxidative stability of PP-g-POSS was explained7,8 by accumulation of POSS on the sample surface, its oxidation, and the production of a ceramic protective barrier. This limits polymer volatilization rate.8 The high residue observed in PP-g-POSS supports that hypothesis.


The dependence of mechanical properties on the composition of PP-POSS hybrid nanocomposites and blends. amb: aminopropylheptaisobutyl. amo: aminopropylheptaisooctyl. am2b: aminoethylaminopropylheptaisobutyl.

We also studied the mechanical properties of the nanocomposite in tensile and Izod impact tests at room temperature. PP-g-MA and PP-g-POSS deformed in a brittle or semi-ductile manner. In contrast, when diluted with iPP (50–75% by weight) they demonstrated a cold drawing behavior, typical for iPP. The elastic modulus of PP-g-POSS hybrids and their blends with iPP is somewhat lower than of plain iPP, see Figure 3(a). A similar, weak decreasing trend is observed for the yield stress, cf. Figure 3(b). Both the modulus and yield stress, being principally a property of the crystalline phase, follow the changes of crystallinity and crystal thickness, respectively. These changes in properties of the crystalline phase are induced mostly by the presence of a large quantity of PP-g-MA compatibilizer and additionally by a small fraction of grafted POSS molecules, all of which reduce PP crystallization ability.

Temperature of the maximum weight loss rate and the residue left at 500°C, determined by thermogravimetric analysis.4

SampleTd in N2Td in airResidue in air
(°C)(°C)(% by weight)
iPP471.9383.80.52
PP-g-MA470.7379.40.73
PP-g-amb POSS (5%)471.8396.52.23
PP-g-amo POSS (5%)471.8399.11.70
PP-g-am2b POSS (5%)472.3415.62.01

We observed a much larger variation in ultimate properties, especially in strain at break, see Figure 3(c). Nanocomposites containing grafted POSS break at a strain that is notably lower than samples without POSS. This large-strain deformation ability is generally a property of the amorphous component, and is controlled primarily by properties of the molecular network of entangled chains.9 The grafting of bulky POSS molecules onto PP should induce noticeable changes in the properties of the amorphous phase as grafted segments lose their ability to crystallize and have to be redistributed into the interlamellar amorphous phase upon solidification. This leads not only to thinner lamellae, but also to a more entangled molecular network within the amorphous phase. Additionally, those segments with POSS bound are slightly less flexible and demonstrate longer relaxation times than unmodified segments.4 All this results in a stiffer molecular network and consequently in notably reduced deformability as compared to iPP. The highest degree of grafting results in the deepest modification of nanocomposites with aminoethylaminopropylheptaisobutyl-POSS (amb2 POSS).

In summary, reactive mixing of iPP and PP-g-MA with amine-functionalized POSS leads to the formation of polymer-inorganic hybrids, in which POSS cages, grafted onto PP chains, are finely dispersed. The modification with POSS does not markedly affect the impact properties of the nanocomposite, although we did observe a small increase in toughness. In addition, PP grafted with POSS demonstrates highly improved thermo-oxidative stability. Grafting results in a small reduction of lamellae thickness and crystallinity, and significant modification of amorphous phase (entanglements, chain mobility). These slightly affect the low-strain properties of hybrid nanocomposites but significantly worsen the ultimate properties, depending on the properties of the amorphous component. In future we will concentrate on the preparation and performance evaluation of similar POSS nanocomposites with polyethylene as a matrix.


Authors

Zbigniew Bartczak
Centre of Molecular and Macromolecular Studies (CMMS) Polish Academy of Sciences (PAS)

Zbigniew Bartczak is an associate professor. His main field of research is related to the structure and properties of crystalline polymers, including micromechanisms of plastic deformation as well as properties of polymer blends and composites.

Magdalena Grala
Centre of Molecular and Macromolecular Studies (CMMS) Polish Academy of Sciences (PAS)

Magdalena Grala is a PhD student at CMMS PAS,Łódź, Poland. Her research focuses on polymer nanocomposites and their properties.


References

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DOI:  10.2417/spepro.004845