Numerical Simulation Of Gelatin Physical Gelation Processes Under Non-uniform Temperature Field

With the development of science and technology, gels have been widely used in people's daily life, and are paid more and more attentions by researchers. Polymer physical gel is a major class of gel material, which has good sol-gel and gel-sol reversible transition property. Due to this special property, polymer physical gels are commonly used as smart materials to achieve certain functions, which are different from polymer chemical gels that are used as structural materials. Whether from the aspect of the development direction of modern science and technology, or from the aspect of the need in people's daily life, polymer physical gels have become very advanced and important materials, and have great scientific and practical application values.Gelatin gel is one kind of polymer physical gel. When the temperature of gelatin is reduced below the equilibrium melting temperature, the gelatin molecule coils will wrap each other to form triple helix structure. These helices act as the crosslinks and connect the molecules to form three-dimensional network structure. When the concentration of triple helices reaches a certain value, the gelatin gel is formed. When the gelatin temperature is increased above the equilibrium melting temperature, the helices will melt again, and the gel will transform into the sol. Attributed to the good features of reversible transition, gelatin is widely used in controlled drug release, biological tissue engineering, photographic, food and cosmetic industries.Plenty of studies on the gelatin physical gel have been carried out by researchers, which include gelation mechanisms, gelation kinetics, analysis on the crosslinked structure and macroscopic performance during the gelation process. In the gelation process of gelatin, the low thermal conductivity of gelatin can lead to the uneven and unstable temperature field, which further results in the structural and performance inhomogeneities, especially when the gelatin is large. The structural and performance inhomogeneities of gelatin will directly affect its application. E.g., as a drug carrier, a gelatin gel with nonuniform crosslinking degree can adjust the rate of drug release. As a biological tissue scaffold, the inhomogeneity of the mechanical property of gelatin can affect the strength and life of the scaffold. The dynamic gelation process makes it difficult to measure the structural and performance parameters in time and space scales in experiments. Hence, the computer simulations are used to analyze the gelation processes of gelatin.In this dissertation, the study on gelation processes of gelatin under complicated temperature field is carried out by finite element method. The evolution characters of crosslinked structure and macroscopic performance during the gelation processes are analyzed. The differences of gel structure and performance on different nodes caused by uneven temperature field are revealed, and the effects of material formulation and processing conditions on gel structure and performance are discussed.The main works and conclusions are as follows.(1) The coil-helix transition kinetic model of gelatin under complicated temperature field is rebuilt on the basis of the kinetic equations. The numerical calculation equations of reverted helix fraction are constructed on the basis of backwards difference method. Each triple helix is considered as a crosslink, the definition of crosslinking density increment is presented, and the numerical calculated equation of the total crosslinking density is constructed. A length distribution density function is introduced to describe the continuous distribution of the triple helix length. The number average helix length, weight average helix length and helix polydispersity index are defined during the calculation.(2) The numerical relationship equations between the specific optical rotation of gelatin and the reverted helix fraction are deduced. The relationship equation between the reverted helix fraction and the initial concentration of gelatin at the gel point is constructed, and the reverted helix fraction value can be used as the criterion to judge the gel point. The viscosity before the gel point, the equilibrium shear modulus and storage shear modulus beyond the gel point are calculated on the basis of dynamic scaling theory.(3) The finite element analytic processes of the discreteness of temperature field, calculation of interpolation function, element variation, total synthesis and boundary conditions are described in detail. The finite element calculated equations of uneven temperature field are obtained regardless of the weak exothermic.(4) The finite element simulation method and specific steps of gelatin physical gelation field are expounded by considering the uneven temperature field. The handling of some key technologies and values are explained, e.g., the discrete methods and rules of gelation fields, selection of heat transfer boundary conditions, selection of gelation temperature, the calculation and determination of thermal physical parameters such as the isopiestic specific heat capacity and thermal conductivity, the determination of rheological indexes and prefactors during the calculation on the performance parameters. The finite element simulation programs of thermal induced gelatin physical gelation processes are developed.(5) The comparisons between the simulation results and the experimental ones under the same gelation conditions show that the values of reverted helix fraction and storage shear modulus of the two kinds of results are very similar, the simulation programs are proved to be correct. A two-dimensional model of a rectangular gelatin with a rectangular core inside is analyzed by the simulation program. Although the boundary conditions are constant, the temperature field formed by the heat transfer is uneven. The simulation results on the evolution of the structural and performance parameters have both scientific and practical significance. The design of changing material formula and processing conditions is suggested on the basis of the simulation results.