| Abstract |
In the present thesis, the effect of macromolecular architecture on thermal and mechanical properties of miscible polymer blends is being studied. In particular the polymer systems examined are blends of linear polyethylene oxide, (PEO) with low molecular weight, Mw = 0.55 kg/mol (LPEO-0.55K), and poly (methyl methacrylate), PMMA, with different molecular weight and macromolecular characteristics. The specific systems have interesting properties depending on the architecture of PMMA (linear chain versus nanoparticles composed of high functionality star-shaped polymers) and they show a great promise for their use as high performance solid polymer electrolytes for lithium batteries. More specifically, blends of linear PMMA chains, LPMMA, with various molecular weights of 50, 100, 225 kg/mol, (LPMMA-50K, LPMMA-100K και LPMMA-225K) were blended at different weight fractions with LPEO-0.55K, as well as the corresponding blends of 3 different polymeric nanostructured nanoparticles consisting of miktoarms star of polystyrene, PS, and poly(methyl methacrylate), PMMA, (PS) n (PMMA) n, where n is the number of arms. The one star has hydrodynamic radius Rh≈45nm while the molecular weight of each PS arm is MwPSLi = 1kg/mol and the molecular weight of PMMA arm is MwPMMA ≈20 kg/mol. The number of arms are approximately n ~ 100 (PS100PMMA100-1K-20K). Τhe other two stars have n ~ 40, MwPSLi = 6 kg/mol while the MwPMMA was either 16 kg/mol ((PS) 40 (PMMA) 40-6K-16K and Rh ≈ 15nm) or 16 kg/mol ((PS) 40 (PMMA) 40-6K-6K and Rh ≈ 11nm). The first part of this thesis reports the effect the weight of the linear PMMA on the degree of crystallinity of LPEO. The degree of crystallinity decreased with the weight fraction of the LPMMA in a fairly linear way, as the results of the miscibility/ chemical compatibility of the components, and no Mw-dependence is reported. The effect of nanoparticle/stars of PMMA size in the degree of crystallinity of LPEO is examined by comparing the blends of LPEO-0.55K/(PS) 40 (PMMA) 40-6K-16K and LPEO/(PS) 100 (PMMA) 100-1K-20K. These two star-shaped polymers/nanoparticles are primarily composed of PMMA, but differ in size. For low weight fraction (wt%) of PMMA nanoparticles, no effect of the size on the decreases of crystallinity is reported and decreases in a similar manner as in the case of LPMMA blends. This behavior shows that despite the large number of PMMA arms (high functionality), the low molecular weight PEO chains can wet the particles and that in the low wt% regime the number of PMMA/LPEO contact dominate the final behavior. Nevertheless, with increasing wt%, a sigmoid, sharp drop in crystallinity is seen that occurs at lower values of wt% for the smaller (PS) 40 (PMMA) 40-6K-16K star/nanoparticles. This phenomenon may be attributed to the fact that the nanoparticles form a percolation network at a wt% larger than a critical, percolation threshold, and this should appear at lower value with decreasing the size of the nanoparticle. Subsequently, the effect of the (PS) n (PMMA) n nanoparticle composition on the crystallinity of LPEO is being studied. For this purpose, the LPEO0.55K/(PS) 40 (PMMA) 40-6K-16K blends are compared with the corresponding LPEO0.55K/(PS) 40 (PMMA) 40-6K-6K blends. The diagram of the degree of crystallinity as a function of the PMMA wt% reveals that a strong drop in the crystallinity of LPEO with increasing wt% for the LPEO-0.55K/(PS) 40 (PMMA) 40-6K-6K blends. In this case, the presence of PS arms with similar length to those of PMMA leads to the formation of the network in smaller weight fractions. The self-assembling of nanoparticles in interconnected structures within PEO is the result of two competing interactions: i) intermolecular attraction between PS arms, as they are non-miscible with LPEO, which benefit the aggregation of nanoparticles (ii) The attractive interactions between the PMMA and LPEO are in favor for their dispersion. Finally, the above systems are being studied in terms of their mechanical properties. The (PS) 40 (PMMA) 40-6K-16K/LPEO blends show better mechanical properties than the corresponding blends of the larger (PS) 100 (PMMA) 100-1K-20K particles, as the results of the formation of a percolation network at 45 wt%. Noticeably, in both cases the mechanical properties of the blends with the PMMA nanoparticles were superior compared to the corresponding LPMMA/LPEO blends indicating the effect macromolecular architecture. Finally, the (PS) 40 (PMMA) 40-6K-6K/LPEO blends, due to the formation of a strong-glassy percolated phase, appear to have the best mechanical behavior showing a strong solid-like behavior at wt% larger than 30%. Differential scanning calorimetry measurements revealed that in the case of (PS) 40 (PMMA) 40-6K-6K particles the thermal stability depends on the PS-phase, and the material departs from its glassy-like behavior at a temperature close to the glass transition temperature of the PS, while when (PS) 40 (PMMA) 40-6K-16K particles are considered, the aforementioned behavior is governed by the glass transition temperature of the PMMA.
|
Search
