(236h) Nanoscale Polymer Blends Via Mechanical Milling

Conventional melt and solution processes cannot overcome some of the technical barriers inherent in blending polymers. In fact, many polymers are immiscible with respect to each other, and the kinetic and thermodynamic limitations of compatibilizers prevent fulfillment of optimal property enhancement by blending. Mechanical milling (MM) provides a solid state route to intimately mix polymers at the nanoscale in order to overcome thermodynamic macroscale phase separation.

The ultimate goal of this research is to compatibilize polymer pairs that are typically immiscible. In order to better understand compatibility and immiscibility, the morphology, crystallinity, molecular weight, and thermal transitions (namely glass transition temperature) are targeted for study. High-energy ball milling has been extensively studied for mechanical alloying by authors such as Koch, Smith, and Torkelson. Cryogenic milling, in particular, has been shown to compatibilize polymers through amorphization.

Preliminary particle size analyses indicated that the SPEX ball milling and cryogenic milling apparati indeed created ultrafine particles (< 100 microns). Cryogenic milling introduced much less contamination than ambient ball milling. Substituted poly(p-phenylene) was used for these studies because of its hardness and properties similar to mild steel. Cryogenic milling also amorphized polymers more than ambimilling. This was evidenced in DSC plots for polyethylene terephthalate (PET), polyethylene oxide (PEO), and polyvinylpyrrolidone (PVP) as well as XRD patterns for PEO and PVP.

Molecular weight studies were done with gel permeation chromotography and light scattering in series (GPC/LS) for water-soluble PEO and PVP to observe any molecular weight changes. The more crystalline PEO undergoes less molecular weight degradation than the more amorphous PVP. PEO is highly crystalline (~75 wt %), and MM reduced crystallinity to 55 ? 62 wt %. PVP is more amorphous, and its crystallinity was less affected by MM (30 to 25 wt %). XRD-derived crystallite sizes from FWHM data agreed with these calculated percent crystallinities: PVP had a crystallite size lower than PEO (1.5 nm vs 23 nm on average) and, unlike that of PEO, it remained almost constant with MM. XRD results also indicate a possible milling-induced lattice transformation for PEO.

Fourier transform infrared spectroscopy (FTIR) analysis of these polymers and their blends was performed, and initial results showed some changes in bond stretches and deformations with milling as well as hydrogen bonding in several blends, particularly PEO/PVP and PPP/PEO. Transmission electron microscopy (TEM) images of the blends show an affinity between the blend components and also reduced domain size for the semicrystalline polymer PEO.