Transparent Polymer-Polyhedral Oligomeric Silsesquioxane Composites

Chapter 20

Transparent Polymer-Polyhedral Oligomeric Silsesquioxane Composites 1

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Downloaded by COLUMBIA UNIV on August 3, 2012 | http://pubs.acs.org Publication Date: August 2, 2007 | doi: 10.1021/bk-2007-0964.ch020

Subramanian Iyer , Amjad Abu-AIi , Andrew Detwiler , and David A. Sehiraldi* 1

Department of Macromolecular Science and Engineering, Case Western Reserve University, 2100 Adelbert Road, Cleveland, O H 44120 Current address: PolyOne Corporation, Avon Lake, OH 44012 *Corresponding author: [email protected] 2

Polyhedral oligomeric silsesquioxanes (POSS ) are an important class of nanoscale inorganic-organic hybrid materials, which have been incorporated into organic polymers via melt blending as well as copolymerization. In this paper, the different criteria necessary for formation of transparent POSS-polymer composites are discussed, as are general concepts governing the thermal/mechanical properties of these composites. A series of nanocomposites were produced from polyhedral oligomeric silsesquioxane materials and four transparent, amorphous, engineering plastics. While the majority of the nanocomposites within this study retained high optical clarity, only cellulose propionate composites exhibited both high clarity and enhanced thermo-mechanical properties. The specific system, which exhibited these desirable properties, is the only one tested which can be expected to possess strong particle-matrix interactions. Such interactions are likely a key to the preparation of stable melt-blended POSS nanocomposites of engineering plastics, which possess enhanced properties.

© 2007 American Chemical Society

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In Science and Technology of Silicones and Silicone-Modified Materials; Clarson, S., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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Introduction Polyhedral oligomeric silsesquioxane (POSS®) molecules possess a cagelike structure (1-3 nm in size) and a hybrid chemical composition (RSiOi ) which is intermediate between silica (Si0 ) and silicones (R2S1O). The welldefined structure of these nano materials contains stable inorganic S i - 0 cores surrounded by substituents which can be modified to present a wide range of polarities and reactivities. POSS molecules can be incorporated into polymer systems through blending, grafting or copolymerization ' ' ' aiming at nanostructured polymeric materials whose properties bridge the property space between organic plastics and ceramics. Recent studies on POSS-containing hybrid copolymers and thermosets have been reported indicating reinforced mechanical ' and thermal properties. " POSS is expected to act as a nano-scale filler with which to modify polymer matrixes and potentially produce nanocomposites with new or improved properties. The organic shell of substituents can be used to create compatibility by matching polarity with host polymers, simultaneously improving mechanical properties due to reinforcement by the rigid silicate core. When effectively incorporated into polymer systems, these nanoscale fillers have been shown to improve the thermomechanical properties of materials that range from polyethylene to epoxy networks, as well as imparting resistance to singlet oxygen and decreased flammability. In this manner, control of the microstructure of the POSS nanocomposites can be achieved and is key to the performance and properties of such materials. It is generally believed that nano-scale POSS domains with ordered and selfassembled features in a polymer matrix are highly desirable and lead to the observed improvement in material properties. Even though POSS has the ability to be dispersed in polymers, most polymer-POSS systems are opaque due to solubility limits and inadequate dispersion domain sizes. POSS grades exhibit drastically different effects on polymers, ranging from plasticization, no effects, to increases in glass transition temperatures. The production of clear transparent composites is desirable for various applications, such as aviation windshields capable of withstanding the high temperatures associated with supersonic speeds. Polyhedral oligomeric silsesquioxanes have the general structure shown in Figure 1. The R groups can range from hydrogen, to bulky isooctyl or phenyl groups and can contain functional groups such as epoxides or isocyanates. Due to their size and potential to be compatible with polymers, POSS fillers have the ability to form true molecular composites; compatibility of these materials with polymers will be governed with the nature of their organic peripheries, while their inorganic cores serve to reinforce the polymers. Though there has been a significant amount of work carried out with POSS copolymers showing enhanced thermomechanical properties of polyurethanes, 5

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epoxy thermosets, and dicylcopentadienes, very little has been reported concerning the use of POSS as a reinforcing filler. In this paper we report melt blending of POSS grades with four amorphous polymers, combinations that were carefully chosen in order to facilitate compatibility and interactions between polymers and fillers. The materials investigated in this paper include amorphous copolyester, a cyclic olefin copolymer, polycarbonate and cellulose propionate. Differences between composites containing fully-condensed POSS cages, and those possessing a hybrid inorganic-organic 3D partial cage-like structure bearing three silanol (Si-OH) groups are also examined. It was recently reported that the trisilanol POSS grades, especially isooctyl trisilanol POSS, exhibit enhanced compatibility with PET, as judged by single phase melts and optically transparent extrudate. Downloaded by COLUMBIA UNIV on August 3, 2012 | http://pubs.acs.org Publication Date: August 2, 2007 | doi: 10.1021/bk-2007-0964.ch020

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Experimental All materials were used as received with any further modification. Polyethylene terephthalate/cyclohexanedimethanol copolymer, PETG, and cellulose propionate (Tenite®, CP) were supplied by the Eastman Chemical Company. Polycarbonate (PC, Makrolon® 2405) was obtained from Bayer AG. The cyclic polyolefin copolymer (COC, Topas® 8007) was obtained from Ticona GmbH. All materials were dried at 100°C 24 hours prior to extrusion. The extrusion temperatures and the different POSS grades used with different polymers are listed in Table 1. Materials were processed on a DACA co-rotating microextruder (Model 20000) with a residence time of 5 minutes. Post extrusion, the extrudates were compression molded at the respective processing temperatures at approximately 12000 lbs pressure. Samples were heated and held under pressure for about 1 minute before quenching the films between water cooled platens. Films of 0.3 mm thickness were made for characterization. The different POSS grades used in this study were of two forms; either a fully condensed cage structure or an incomplete cage structure with silanol groups. The representative structures for the two forms of POSS are shown in Figure 1. The blended samples were examined for qualitative differences in transparency, color and toughness. Dynamic mechanical analysis was performed on a Tritec-2000 dynamic mechanical analyzer from Triton Technologies, UK. Scanning electron microscopy was carried out on a Phillips X-30 ESEM, after coating fractured surfaces of films with palladium. An Instron model 5565 universal testing machine was used to obtain the tensile properties of the samples. Testing was conducted at three temperatures, 23, 85 and 120°C; a strain rate of 12.70 mm/min was used for these measurements (5 specimens of each sample were tested). Results are reported for the high temperature runs only.



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Figure 1: Representative structures of complete and incomplete cage structures of POSS

Table 1: Processing temperatures and POSS grades used with different polymers Polymer PETG COC

PC CP

POSS grades Trisilanol Isooctyl POSS Isooctyl POSS Isobutyl POSS Trisilanol Isobutyl POSS Trisilanol Isooctyl POSS Trisilanol Phenyl POSS Isooctyl POSS Trisilanol Isooctyl POSS Trisglycidyl isobutyl POSS

Processing Temperature °C 240 220

290 200


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Results


Appearance The extrusion observations and sample appearances of compression molded polymer/POSS films are summarized in Table 2. In the case of PC/POSS blends, clear composites were obtained only with phenyl trisilanol POSS, while trisilanol isooctyl POSS and octaphenyl POSS both resulted in opaque extrudates. The compression molded films of PC/phenyl trisilanol POSS exhibited a high level of transparency, while the other PC blend films were translucent. It is reasonable to conclude then that only phenyl trisilanol POSS was well dispersed within a polycarbonate matrix. Each of the POSS grades melt blended with COC produced transparent clear films and extrudates, indicating high levels of compatibility/miscibility between these materials. Since there is no likelihood of chemical interactions between the hydrocarbon COC polymer and the POSS grades, we therefore expect the nanoparticles to be molecularly or nano-scale dispersed within the COC copolymer system. When PETG was used as the polymer matrix, opaque extrudates and translucent films were obtained. The absence of compelling polarity matches, and the possibility of POSS thermal degradation under polyester processing conditions apparently lead to poor dispersion in these cases. The appearance of transparent, ductile cellulose propionate was largely unaffected by addition of the POSS grades tested, up to as much as 5 wt% filler, indicating significant compatibility for these systems.

Dynamic Mechanical Analysis and Tensile Testing Figures 2-5 illustrate the effects of POSS grades on the thermomechanical properties of the four transparent engineering plastics examined in this study. These DMA results can be classified into three distinct behaviors. In the first case, there is a clear plasticization effect as observed by reduction in the glass transition temperatures and moduli of the composites as various POSS grades are added; both polycarbonate and the cyclic olefin copolymer exhibit this behavior. In the second case, the addition of POSS to the glycol modified polyethylene terephthalate has no measurable effect on the glass transition temperature and little on the modulus of the composites. Reinforcement is observed as the third type of behavior - this effect is seen with incorporation of POSS in cellulose propionate. The rubbery modulus of the CP composites increases 100 fold with incorporation of POSS at 5 wt%, while there is no significant shift in the glass transition temperature. Consistent with reinforcement/enhanced rubbery modulus of CP upon addition of POSS, its high temperature tensile modulus increases with filler loadings up to 10 wt%, Figure 6.



Compression molded film observation Transparent, Tough Translucent, Tough Translucent, Tough Transparent, Ductile Transparent, Ductile Transparent, Brittle Transparent, Ductile Transparent, Ductile Transparent, Brittle Translucent, Ductile Translucent, Bubbles, Brittle Transparent, Ductile Transparent, Ductile Transparent, Ductile

Extrusion Observation Transparent, Clear Opaque Opaque Transparent, Clear Transparent, Clear Transparent, Clear Transparent, Clear Transparent, clear Transparent, clear Opaque Opaque Transparent, Clear Transparent, Clear Transparent, Clear

Composition

PETG control 5% Isooctyl POSS 5% Trisilanol Isooctyl POSS

COC control 5% Isobutyl POSS 5% Trisilanol Isobutyl POSS 5% Trisilanol Isooctyl POSS

PC control 5% Trisilanol Phenyl POSS 5% Trisilanol Isooctyl POSS 5% Octaphenyl POSS

CP Control 5% Trisilanol Isooctyl POSS 5% Trisglycidyl Isobutyl POSS

Table 2. Extrusion observation of various Polymer/POSS composites



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Figure 2: DMA curves for PETG composites

l.OE+10

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•COC Control COC-Octaisobutyl POSS COC-Trisilanoi Isobuty POSS1 COC-Trisilanol Isooctyl POSS

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Figure 3: DMA curves for COC composites


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-PC control -5% Trisilanol Phenyl POSS -5% Trisilanol Isooctyl POSS

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Temperature °C Figure 4: DMA curves for PC composites

1.0E+10

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-CP control - 2.5% Trisilanol Isooctyl POSS - 5% Trisilanol Isooctyl POSS . 10% Trisilanol Isooctyl POSS .OE+07 40

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Temperature (°C) Figure 5: DMA curves for Cellulose Propionate composites


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Figure 6: CP/isooctyl trisilanol POSS Tensile testing, 120°C

Scanning Electron Microscopy The morphology of one of the highly compatible systems, polycarbonate with 5% phenyl trisilanol POSS blends is shown in Figure 7. In case of such PC/POSS blends, few if anyfiller-relatedfeatures are evident. Analysis by EDA X confirms that almost equal concentrations of silicon exist between matrix and features (such as they are), consistent with miscibility within the PC/POSS blends.

Discussion and Conclusions The compatibility or miscibility of POSS particles within matrix polymers clearly depends upon the nature of the R groups which decorate its corners; in some cases, these peripheral organic groups can serve to shield the silicate core from the organic hosts. Incomplete ("T7") POSS cage structures possess three free silanol groups, which are capable of interacting with carbonyl groups as well as hydroxyl groups on the polymer via hydrogen bonding. As a viscous fluid with high viscosity, trisilanol isooctyl POSS and isooctyl POSS grades are different from the crystalline POSS materials examined in this study. It has also been demonstrated that although POSS is relatively stable up to 300°C, given sufficient time and temperature, some degree of nanofiller degradation is 17



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Figure 7: SEM micrograph of a) Polycarbonate and b) 5% phenyl trisilanol POSS


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possible. Such degradation can result in an increase in the molecular weight of POSS thus causing aggregation/oligomerization of particles, increasing the possibility of phase separation between thesefillersand matrix polymers. From the DMA results presented herein, a decrease in the glass transition temperatures of polycarbonate and cyclic olefin copolymers was observed. The results from SEM and EDAX further show that in case of polycarbonates, POSS is molecularly dispersed. It has been demonstrated that when POSS is molecularly distributed in polymers there is often a reduction in the glass transition temperature of the composite. We believe that when POSS is molecularly dispersed in the system, individual particles act as molecular lubricants, allowing for chain slippage in the polymer. This slippage results in lowering of the glass transition temperature of the polymer. However, when the compatibility between the polymer and POSS is low, as in the case with PETG, there is no significant effect on the glass transition temperature. Lower compatibility between POSS and the polymer leads to larger particle size of the dispersed phase, which results in opaque/translucent composites. Since there is gross phase separation between the phases, POSS does not significantly affect the polymer properties. The third system investigated was the cellulose propionate/trisilanol isooctyl POSS system. Cellulose propionate possesses a large number of hydroxyl groups capable of interacting with the silanol groups on incompletely condensed POSS cages. The hydrogen bonding interactions between the filler and the polymer can exist beyond the glass transition temperature of the polymer, thereby causing an increase in rubbery modulus and high temperature tensile properties. Hence, transparent nanocomposites of POSS and amorphous engineering plastics can be obtained in cases where high levels of compatibility exist. Simple solubility of nanofiller in polymer plasticizes the transparent plastic, similar to the action of more traditional organic plasticizers. In such cases when polarities of host and guest are well matched, with additional polymer-filler interactions (such as hydrogen bonding), molecular scale reinforcement of the polymer can also be obtained without loss of optical clarity.


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Acknowledgements Financial support from Hybrid Plastics, and gifts of polymers from Ticona, Bayer and Eastman are gratefully acknowledged.

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Transparent Polymer-Polyhedral Oligomeric Silsesquioxane Composites

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