(246f) Self Assembly of POSS and Sorbitol and Their Effects On the Reinforcement of Polypropylene Spun Fiber

Abstract:

This
study developed fundamental understanding of nanocomposite formation by ?bottom-up?
self-assembly of inorganic-organic hybrid molecules (POSS) in isotactic polypropylene
(iPP) matrix. A sorbitol nucleating agent (DBS) compatible with POSS with
silanol (Si-OH)
functionalities promoted dispersion of POSS molecules and
provided templates for self-assembly into cylindrical nanoparticles in the spun
fibers. It was found that at the optimum combination of phenyl-POSS and DBS, iPP
composites can be spun into fibers at higher draw-ratios compared to unfilled polymer
and provided superior mechanical properties. Rheological studies revealed
phenyl silanol-POSS prevented fibrillation of DBS. The interactions between DBS
and silanol POSS with various functionalities were investigated using infra-red
spectroscopy (FT-IR), differential scanning calorimetry (DSC), wide-angle X-ray
diffraction (WAXD), nuclear magnetic resonance spectroscopy (NMR), mass
spectrometry (MS), and their synergistic morphology by scanning electron
microscopy (SEM). The mixtures of phenyl silanol-POSS with DBS when heated at
the processing temperature of iPP (200°C) formed a low viscous transparent
liquid. The cylindrical shaped nanoparticles as found in spun fibers were
believed to have resulted from the stretching of this liquid complex. No
chemical reaction occurred between DBS and POSS as identified from the solid
state NMR and complexes of different molecular ratios were identified from mass
spectrometry study. It was inferred that a combination of hydrogen bonding
interactions and intermolecular π-π stacking between the phenyl rings
played a major role in the formation of supramolecular complexes. Higher orientation
of polymer chains at higher draw-ratios was identified to be the possible cause
of mechanical strength improvement.

1.     
Introduction

Our study focused on dispersion of polar, crystalline polyhedral
oligomeric silsesquioxane (POSS) molecules of 1-2 nm size in non-polar polyolefins to obtain
large enhancements in mechanical properties. The incompatible POSS molecules do not disperse
well and remain as agglomerates of 10-20 mm size in polyolefins.[1] Such large size POSS particles act as defects and
deteriorate mechanical properties.[2] In this context, POSS with non-polar side groups
disperses well in polyolefins, but do not contribute much to the properties of
the molded composites due to greater plasticization effects.[2, 3] So the scientific challenge of the work is to find a dispersion
aid that can produce nano-scale dispersion of an incompatible POSS in non-polar polyolefins and
having a subsequent improvement in mechanical properties.

This
research exploited the fundamentals of nanocomposite formation by a ?bottom-up?
self-assembly approach from dispersions of POSS molecules in polyolefins. These
composite materials had the processing ease of unfilled polymers and were
suitable for manufacturing of articles with micro- and nano-scale features by
high-speed fiber spinning. The fundamentals developed in this work offered
superior alternatives to most ?top-down? polymer nanocomposites, where orders
of magnitude increase in viscosity over that of the host polymers is a norm and
achieving nanoscale dispersion is a challenge. The degree of nanofiller/polymer
and nanofiller/nanofiller interactions was governed by the choice of polymer and
nature of the side groups of POSS.[1, 4, 5]

In our work, non-reactive POSS containing
multiple silanol groups were dispersed in iPP with the aid of sorbitol-type
nucleating agent, di-benzylidene sorbitol (DBS). The
underlying chemistry between DBS and POSS
ramified some unique molecular structures in molten PP, which in turn led to
nanoparticle formation in the final stages of fiber spinning and provided
mechanical property improvement.  

2.     
Materials and methods

Various
grades of POSS were chosen ? tetra-silanol phenyl-POSS (tetra-POSS),
tri-silanol phenyl-POSS (tri-POSS), tri-silanol cyclopentyl-POSS (cyclo-POSS),
and tri-silanol isobutyl POSS (iso- POSS). These were obtained from Hybrid
Plastics. The sorbitol derivative chosen for this study was
di(benzylidene)sorbitol (DBS) and was obtained from Milliken Chemicals. For
composite preparation, isotactic polypropylene (iPP), P4G2Z-159, was obtained
from Flint Hill Resources (MFI 1.95 g/10 min, T_m 165°C). The materials were mixed in a batch mixer at 200ºC for 5
minutes to obtain compounds.  In some compounds the nucleating agent DBS was
not used to determine the potential of nanoparticle formation by POSS molecules alone. An
optimum ratio of DBS and POSS were determined using rheological study for further
processing. The optimized polymer composites were subsequently melt-spun into monofilament
fibers using a capillary rheometer (Instron) and a take-up device. The
values of tensile modulus, tensile strength and yield strength of the fibers were determined. The fibers were characterized by DSC and
WAXD and morphology by SEM.

The synergistic interactions between POSS and DBS molecules
were studied using FT-IR, DSC, and MS and any possible reaction by solid state
NMR. DBS/POSS
self-assembled structures interdependent of iPP were observed by SEM.

3.      Results and discussion

The POSS molecules with multiple silanol groups were selected so
that non-covalent hydrogen bonding interactions can occur with the free
hydroxyl groups of DBS which was identified by FT-IR. DBS is known to
form 3-D fibrillar networks in PP melt.[6]  However, the supramolecular complexes between phenyl silanol-POSS and DBS suppressed such fibrillar network formation as revealed from oscillatory shear rheology.[7] For cyclo- and iso-butyl POSS, the DBS fibrillation was still visible which suggests insufficient interactions between non-phenyl POSS and DBS. This trend was interpreted by invoking the possibility of π-π stacking between the phenyl rings. It was confirmed by molecular dynamics simulation that DBS/tri-POSS and DBS/tetra-POSS molecules first engaged in π-π stacking and then formed hydrogen bonding leading to the formation of mutlimolecular complexes, which in turn subdued DBS fibrillation completely.  Such π-π stacking was not possible in the case of cyclo-POSS and iso-POSS with DBS system. In corroboration with the rheology data, the optimum combinations of DBS with tri- and tetra-POSS in PP also produced lowest crystallinity. In this case, the nucleating function of DBS[8] was arrested due to participation of DBS in DBS-POSS multi-molecular complexes. Insufficient interaction of cyclo-POSS and iso-POSS with DBS also reflected in no such changes in crystallinity with increasing POSS content.

Tri-POSS
and tetra-POSS formed crystalline lamella structures through self-aggregation via
intermolecular hydrogen bonding between the silanol POSS molecules. In the presence
of DBS, the phenyl silanol-POSS formed much smaller crystalline aggregates and
distributed uniformly into the DBS network (Figure 1). On the other hand, cyclo-POSS and iso-POSS retained their circular and lamellar domain aggregates respectively, when mixed with DBS. The nature of assembly of cyclo-POSS and iso-POSS indicates greater POSS-POSS affinity,
unlike phenyl silanol-POSS, which showed greater affinity towards DBS.

The
interactions towards the formation of supramolecular complexes between POSS and
sorbitol derivatives at various molecular ratios were also systematically
investigated using WAXD, NMR and MS. No
chemical reaction occurred between them as identified from the solid state ²⁹Si
and ¹³C NMR and supramolecular complexes of different molecular
ratios were identified from MS ? Electrospray
Ionization study. Only complexes with 1:1 molecular ratio were
identified between cyclo-POSS and DBS, whereas higher molecular ratio complexes
were observed for tri-POSS and tetra-POSS with sorbitol derivatives.

Fine
fibers were obtained with DBS/tri-POSS and DBS/tetra-POSS compounds. PP
composite formulations with 2-5 wt% of POSS were spun into fibers with close to
40% reduction in diameter. These fibers offered 60-80% increase in modulus,
50-60% increase in tensile strength, and 100% increase in yield strength
compared to unfilled iPP. On the other hand, in spun fibers, the domain size of POSS particles were found to be cylindrical with close to 100 nm in diameter
and 200-300 nm in length (Figure 2). The complex materials formed between DBS and phenyl silanol-POSS were liquids with low viscosity which enhanced the melt-spun fiber to be drawn at very high draw-ratio, where the liquid droplets stretched and deformed
to form the cylindrical nanoparticles. Maximum tensile properties were identified
due to increased draw-ratio of the fibers, which in turn produced finer fibers.

4.      Conclusions

The following conclusions can be drawn from the results
obtained in this study.

·       
PP/DBS/POSS ternary blend system provided the best reinforcement
of PP at its optimum combination with maximum draw-ratio of spun fiber.

·       
The spun fiber allowed formation of self-assembled cylindrical nanoparticles
of POSS/DBS complex.

·       
A combination of hydrogen bonding interactions and intermolecular
π-π stacking between the phenyl rings played a major role in the
formation of supramolecular complexes between DBS and POSS molecules.

Figure 1. Uniformly dispersed POSS
nanoparticles on DBS fibrils (solution cast)

Figure 2. Cylindrical
nanoparticle formation in iPP spun fiber

5.
Acknowledgements:

Financial
support from National Science Foundation in the form of award CMMI-0727231 is
gratefully acknowledged.

6.
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