Electrospinning of linear homopolymers of poly(methyl methacrylate): exploring relationships between fiber formation, viscosity, molecular weight and concentration in a good solvent

A series of seven linear homopolymers of poly(methylmethacrylate) ranging from 12,470 to 365,700 g/mol M_w, were utilized to further explore scaling relationships between viscosity and concentration in a good solvent at 25 °C and to investigate the impact of these relationships on fiber formation during electrospinning. For each of the polymers investigated, chain dimensions (hydrodynamic radius and radius of gyration) were measured by dynamic light scattering to determine the critical chain overlap concentration, c^*. The experimentally determined c^*, was found to be in good agreement with the theoretically determined value that was calculated by the criteria c^*∼1/η], where the intrinsic viscosity was estimated from the Mark-Houwink parameters, K and a (at 25 °C in dimethyl formamide) obtained from the literature. The plot of the zero shear viscosity vs. c/c^* distinctly separated into different solution regimes, viz. dilute (c/c^*<1), semidilute unentangled (1<c/c^*<3) and semidilute entangled (c/c^*>3). The crossover between semidilute unentangled and semidilute entangled regimes in the present investigation occurred at c/c^*∼3, which, therefore, marked the onset of the critical chain entanglement concentration, c_e, according to the procedure utilized by Colby and co-workers Colby RH, Rubinstein M, Daoud M. J de Phys II 1994;4(8):1299-310. 52]]. Electrospinning of all solutions was carried out at identical conditions to ascertain the effects of solution concentration, molecular weight, molecular weight distribution and viscosity on fiber formation and morphological features of the electrospun material. Only polymer droplets were observed to form from electrospinning of solutions in the dilute concentration regime due to insufficient chain overlap. As the concentration was increased, droplets and beaded fibers were observed in the semidilute unentangled regime; and beaded as well as uniform fibers were observed in the semidilute entangled regime. Uniform fiber formation was observed at c/c^*∼6 for all the narrow MWD polymers (M_w of 12,470-205,800 g/mol) but for the relatively broad MWD polymers (M_w of 34,070 and 95,800 g/mol), uniform fibers were not formed until higher concentrations, c/c^*∼10, were utilized. Dependence of fiber diameter on concentration and viscosity was also determined, viz. fiber dia∼(c/c^*)^3.1 and respectively. These scaling relationships were in general agreement with that observed by Mckee et al. McKee MG, Wilkes GL, Colby RH, Long TE. Macromolecules 2004;37(5):1760-67. 33]].