Melt processing of poly(L-lactide) (PLLA) and poly(methyl methacrylate) (PMMA) was conducted over a targeted range of compositions with PLLAs of 118 and 316 kDa in molecular mass to identify morphologies and the phase relationships in these blends. These blends are of interest for use in biomaterials and the morphologies are critical for tissue-engineering studies where biodegradability, pore connectivity and surface texture control tissue viability and adhesion. Simple extrusion of the two polymers produced multiphase blends with an average domain size near 25 microm. Scanning electron microscopy and dynamic mechanical analysis demonstrated that these blends are immiscible, at least in a metastable sense, and regions of co-continuous structures were identified. Such co-continuous, which occurred generally in accordance with rheology prediction models, exhibit a fine interconnected structure that appears effective for fabricating certain biomaterials. A broad and unexpected transition appears in these blends, as measured by modulated differential scanning calorimetry between 70 and 100 degrees C, which may be the glass transition temperature of an alloy phase. The magnitude of this transition is greatest in the fine-structured co-continuous composition region of blends, suggesting the presence of a complex or other derivative of the two primary phases.
Uncompatibilized immiscible blends of polystyrene (PS) and high-density polyethylene (HDPE) were melt-processed in a single-screw extruder fitted with a fine screen mesh and capillary die and were further drawn into filaments to produce near-nanoscale immiscible domains. The resultant morphologies and mechanical properties were studied for these structures in which load transfer is achieved solely by mechanical linkages between blend domains. The morphology of the blends revealed co-continuity approximately in the range of 45-47 volume percent PS. The development of a three-dimensional co-continuous network in 45 vol% PS, as revealed by morphology observations, was also related to a decrease in extruder output rate in this region, an indicator of the melt interaction of the two phases as co-continuity is achieved. Image analysis revealed submicron fibrillar structures near the phase inversion composition where domain sizes ranged from 6-220 nm with an average domain size of 90 nm. Tensile modulus increased with increasing PS content (E ¼ 2.7 GPa at 47% PS) over the entire blend range with values greater than the rule of mixtures up to 50% PS. Strain to failure did not seem to be influenced by co-continuous morphologies and the fine dispersion of PS domains appears to constrain the fundamentally high strain of HDPE.
Melt processing of binary immiscible polymer systems has been a focus of our group as an economical and scalable route to achieve synergistic or superior mechanical properties at and around the co-continuous region without the need of compatibilization. System of poly(L-lactide) (PLLA) and poly(methyl methacrylate) (PMMA) was selected to target bio-related applications, including bone fillers and scaffolds, where the biodegradability of PLLA will enable the integration of native tissue into the material over time. Tunable properties such as morphology, interconnectivity, resorbability and interfacial bonding control the long-term integrity of the new material and influence the interaction and integration of new tissue. Binary blends of PLLA and PMMA has been prepared and characterized over a large range of compositions in which regions of co-continuity are of special interest. Such regions exhibit a well interconnected structure that ensures controlled release of resorbable PLLA. Modulated differential scanning calorimetry (MDSC) detected a broad and unexpected transition between 70 °C and 100 °C. The magnitude of this transition is greatest within co-continuous regions, suggesting the presence of a complex or other derivative of the two primary phases. This complex appears to provide a degree of compatibilization between the phases, thus inducing mechanical property synergism which has been confirmed by flexural and nano-indentation analyses.
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