Photopolymerization-induced crystallization has been demonstrated in blends of polyethylene oxide-diacrylate at temperatures above the depressed melting temperature of the crystalline component. Upon exposure to ultraviolet irradiation, the melting transition curve moves upward and eventually surpasses the reaction temperature, thereby inducing phase separation as well as crystallization. The present paper demonstrates the occurrence of directionally solidified interface morphologies of polymer crystals subjected to a photointensity gradient. The epitaxially grown seaweed or degenerate structures were observed at the circumference (low-intensity region) while the dense branched spherulites developed at the core high-intensity region.

Phase diagrams of blends of poly(ethylene oxide) (PEO)/diacrylate (DA) monomer have been established by means of differential scanning calorimetry and optical microscopy. A phase field theory based on the combination of the phase field free energy of crystal solidification and the Flory-Huggins theory for liquid-liquid phase separation has been developed that is capable of predicting various coexistence regions of the binary crystalline PEO/DA blends, viz., isotropic (I), the coexistence of crystal + liquid (Cr 1 + I), crystal + crystal (Cr 1 + Cr 2 ), and the single phase crystal (Cr) regions. These aforementioned coexistence regions have been further verified experimentally by probing the spatiotemporal emergence of crystalline structure and phase morphology. Guided by these established phase diagrams of the PEO/diacrylate blends, photopolymerizationinduced crystallization experiments have been carried out at the isotropic temperatures slightly above the depressed melting points of PEO crystals. Of particular interest is the development of spherulites in the continuum of isotropic in the crystal + liquid coexistence region, whereas viscous fingering (or fractal growth) occurs during photopolymerization at a higher temperature, showing phase-separated domains within these advancing viscous fingering structures.

The present article describes experimental and theoretical investigations of miscibility and crystallization behavior of blends of poly(ethylene oxide) (PEO) and triacrylate monomer (TA) using differential scanning calorimetry and optical microscopy. The PEO/TA blends manifested a single T(g) varying systematically with composition suggestive of a miscible character in their amorphous states. Moreover, there occurs melting point depression of PEO crystals with increasing TA. A phase diagram was subsequently established that exhibited a solid+liquid coexistence region bound by the liquidus and solidus lines, followed by an upper critical solution temperature (UCST) at a lower temperature. The emerging phase morphology was investigated to verify the coexistence regions. Upon photopolymerization in the isotropic melt above the melting point depression curve, both the UCST and the melting temperatures move upward and eventually surpass the reaction temperature, resulting in phase separation as well as crystallization of PEO driven by the changing supercooling, i.e., the thermodynamic driving force. Of particular interest is the interplay between photopolymerization-induced phase separation and crystallization, which eventually determines the final phase morphology of the PEO/TA blend such as crystalline lamellae, sheaf, or spherulites in isotropic liquid, phase separated domains, and viscous fingering liquids.

Effects of light intensity gradient on development of intricate hierarchical morphology of semicrystalline polyethylene oxide (PEO) and photoreactive diacrylate (DA) blends undergoing photopolymerization-induced crystallization have been demonstrated experimentally and theoretically. The melting temperature of PEO was found to decline upon addition of DA monomer. A solid-liquid phase diagram has been established by self-consistently solving the combined phase field free energy of crystal solidification and Flory-Huggins (FH) free energy of liquid-liquid demixing. Dynamic calculations were performed using time-dependent Ginzburg-Landau (model C) equations by incorporating the combined phase field and FH free energy densities coupled with the photopolymerization kinetics. The spatiotemporal development of gradient morphology was computed under various intensity gradient profiles including linear, cylindrical, and Gaussian profiles. The observed seaweed or dense lamellar branching morphology of the PEO/DA blend is strikingly similar to the directionally grown interface structures observed in metals driven by external thermal gradients.