| Spherulite is a common and important type of crystal form in crystalline polymers. Lamella radially grows outward from a crystal nucleus into a spherulite. Spherulites mostly exist in bulky materials, from which they can hardly be separated using chemical or physical methods. Free PLLA spherulites, PLLA porous microspheres and nylon-6 (PA) spherulites were prepared by thermally induced phase separation (TIPS) combined with self-emulsification and non-solvent phase separation method. The main results are listed below:1) Free PLLA spherulites were fabricated by quenching PLLA/acetonitrile (AN) solutions to the metastable state, resulting in liquid-liquid phase separation in the polymer solutions, followed by nucleation and crystal growth in the polymer rich phase. Through controlling the quenching time, the growing process of free PLLA spherulites was observed, namely the PLLA morphology changed from small sheaves to big sheaves (cauliflower-like) then to spherulites. Meanwhile, with increasing quenching time, PLLA spherulites transformed from a’-crystal to a-crystal (it is a more stable and compact-packing crystal form). When the quenching time and temperature were kept constant, solutions with low PLLA concentrations were favorable for the formation of sheaves, whereas solutions with high PLLA concentrations were inclined to form spherulites.While using AN as a solvent, the film formation ability by PLLA spherulites was poor and there were irregular particles appeared at the end of fibers on spherulites surface. Using tetrahydrofuran (THF) as a solvent, PLLA spherulites showed good film formation ability and the fibers on spherulite surface became smoother and more uniform. The radius of sheaves and spherulites increased from 11.21±2.43 to 24.59±3.62 μm with increasing quenching time from 30 to 90 min. Moreover, the radius of sheaves and spherulites increased from 10.36±1.32 to 34.00±8.70 μm with increasing concentration from 3 to 7 wt%. Low PLLA concentrations were favorable for the formation of sheaves, whereas solutions with high PLLA concentration were good for the formation of spherulites.PA spherulites composed of nanofibers, which radially grow from the microsphere core, were fabricated by combining TIPS with non-solvent induced phase separation. The effect of extractant, concentration, quenching temperature and quenching time on the morphology of PA spherulites was investigated. The results showed that crude fibers/ particles and spherulites were obtained with quenching time of 0-15 and 60-240 min, respectively. Moreover, spherulites became more uniform and regular, and the diameter increased from 19.14±3.62 to 30.53±3.71 μm with increasing quenching time. While quenching at 5 and 10 ℃ was favorable for the formation of fibrous or bud-like or sheaves, quenching at 15 ℃ was inclined to form spherulites. When increasing solution concentration from 3 to 7 wt%, the spherulite diameter showed a rising trend from 15.54±2.11 to 23.21±3.32 μm. The crystallinity of PA spherulites reached up to 62.1%.Except for the free PLLA spherulite and PA spherulites, linear polymers such as PC (polycarbonate) and POM (polyformaldehyde) were prepared by TIPS. Fibrous, sheaves, platelet and spherulites were fabricated by controlling various parameters such as solvent, concentration, quenching temperature and quenching time. Free spherulites provided a new form of porous materials, and provided techniques and mechanism for preparation of other crystalline or non-crystalline polymer spherulites.2) The mechanism for the formation of PLLA sheaves or spherulites was examined by the isothermal and non-isothermal crystallization of PLLA/THF solutions using DSC. The Avrami equation was used to analyze the data and good linear double-logarithmic plots were obtained. In the isothermal crystallization process, the average n and k value increased from 2.12 and 0.38 min -n to 2.29 and 1.68 min -n with increasing concentration from 3 to 7 wt%, indicating that the crystallization rate increased with increasing concentration and the crystallization mechanism was heterogeneous nucleation and 3D growth. For example, in 7 wt% PLLA/THF solution, using Lauritzen-Hoffman theory, crystallization regime changed from regime Ⅲ to regime Ⅱ, and nucleation parameter and Kg(Ⅲ) was 0.44×105and 0.19×105K2, respectively. In the non-isothermal crystallization process, the apparent Avrami exponent n was ca.2.93. Ozawa analysis failed to describe the crystallization behavior of PLLA in the THF solutions. However, Ozawa exponent m=2.29 was obtained by combining Avrami-Ozawa equations. With increasing concentration from 3 to 7 wt%, isothermal crystallization activation energy (Arrhenius method) decreased from -141.13 to -163.68 kJ/mol, and non-isothermal crystallization activation energy (Kissinger method) decreased from -97.19 to -110.49 kJ/mol, indicating that the crystallization process of PLLA in THF solutions was a barrierless and spontaneous process and the higher concentration, the easier crystallization. This investigation would provide some insights into the crystallization behavior of PLLA in solutions.3) The adsorption capacities of PLLA spherulites porous membranes toward silicone oil, dodecanol, paraffin oil and engine oil and oil-water separation properties were investigated. The water contact angle of PLLA spherulites porous membranes was 141.8°, which was about 70° higher than that of the solvent-cast film. Their surface areas and porosity reached up to 13.9 m2/g and 92.1%. The uptake capacity of the porous spherulites membranes for four types of oils were greater than 12 g/g, which were 14-19 times that of solvent-cast film. Owing to the high hydrophobicity and oleophilicity and massive capillary action, the PLLA spherulites porous membranes can quickly absorb oil in preference to water in an oil-water mixture. Such a material is an eco-friendly oil-water separation material.4) Through dispersing the PLLA/THF solution in glycerol, a series of PLLA porous microspheres composed of sheave-like nanofibers were fabricated through a method which combines the self-emulsification with TIPS. Various structures and morphologies of the microspheres were fabricated by controlling PLLA concentration, solvent, ratio of (PLLA/THF):glycerol, and the quenching method. The results indicated that the microspheres with regular shapes were synthesized when the concentrations were 2-5 wt%, the (PLLA/THF):glycerol was 1:3 and quenching at -20 and-196 ℃. The porosity and the specific surface area of the microspheres can reach 95.44% and 32.53 m2/g, respectively. The load capacity of PLLA microspheres for bovine serum albumin (BSA) was 0.355 mg/mg and the drug releasing rate within 30 hours was 59.8%, which reflect that this kind of microspheres is a great drug carrier for sustaining release. |
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