Melt-Quench Formed Smectic Phase in iPP/CNT Nanocomposites and its Re-Crystallization
- PDF / 532,488 Bytes
- 5 Pages / 432 x 648 pts Page_size
- 2 Downloads / 168 Views
Melt-Quench Formed Smectic Phase in iPP/CNT Nanocomposites and its ReCrystallization Georgi Georgiev1,2, Yaniel Cabrera2, Lauren Wielgus2, Scott Schoen2, Devin Ivy2 and Peggy Cebe2 1 2
Department of Natural Sciences, Assumption College, Worcester, MA 01609, U.S.A. Department of Physics and Astronomy, Tufts University, Medford, MA 02155, U.S.A.
ABSTRACT The largest commercial application of carbon nanotubes (CNT) are their polymer nanocomposites (PNCs). This motivates detailed studies of the interactions between CNTs and polymers and the ways the CNTs influence the crystallization behavior and structure of PNCs. We have chosen isotactic polypropylene (iPP) as one of the best model systems because of its ability to form a smectic liquid crystal phase. We studied iPP/CNT nanocomposites with CNT concentrations ranging from 0.01 to 5% per weight. We prepared films by compression molding and quenching in a mixture of isopropyl alcohol and dry ice. Some of the formed smectic liquid crystalline (LC) phase of iPP/CNT PNCs persisted to temperatures higher than the last melting temperature for iPP crystals. By means of differential scanning calorimetry we corroborated the existence of the LC phase in the nanocomposites and studied the impact of multi-walled carbon nanotubes on its melting and re-crystallization. We used a polarizing optical microscopy to reconfirm our DSC data. INTRODUCTION Isotactic polypropylene (iPP) is one of the best model systems to study in the field of polymer nanocomposites because iPP/CNT PNCs can form a liquid crystalline smectic phase and alpha, beta, and gamma crystallographic phases under a variety of crystallization conditions [1]. We prepared iPP/CNT nanocomposites from solution with CNT concentrations ranging 0.01 5% per weight by co-precipitation from a non-solvent. Films were made by compression molding and quenched in a mixture of isopropyl alcohol and dry ice at -70°C. The quenching of the samples induces a smectic liquid crystalline (LC) phase [1]. One of the questions addressed in this study are: How the CNTs affect the smectic phase of iPP and subsequent recrystallization? We found that the perfection of the smectic phase is slightly hindered by measuring a decrease of the melting temperature of the melt-quench formed smectic phase. On the other hand we found that the re-crystallization is improved by measuring a downshift of the exotherm corresponding to this transition. The significance of this downshift is that the CNTs surfaces serve as nucleation centers and induce crystallization at lower temperatures. Similar result on increasing crystallization rates we have observed before in iPP/CNTs PNCs in the absence of smectic phase [3]. The melting temperature of thus formed re-crystallized iPP samples was not affected significantly by the presence of CNTs. CNTs form liquid crystal phase in their lyotropic solutions [4] and interact strongly with liquid crystal molecules [5,6,7]. In our research we are exploring whether the CNTs interact in a similar way with polymer liquid crystalline
Data Loading...
