FINITE ELEMENT SIMULATION OF THE PHYSICAL GELATION OF RENNET CASEIN
FINITE ELEMENT SIMULATION OF THE PHYSICAL GELATION OF RENNET CASEIN
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DOI：
10.3724/sp.j.1105.2008.00529
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发表时间：
2008
影响因子：
1.9
通讯作者：
Feng Ligang
中科院分区：
文献类型：
--
作者：
Feng Ligang
物理凝胶是由聚合物链通过物理结点交联而成的三维网络结构，它既可以通过相变形成，也可以通过某种特定的分子缔合或缠结而形成。由于其内部结构和凝胶化机理的复杂性，很难有目的地获得具有特定组成和性能的高分子物理凝胶材料。聚合物物理凝胶的可控制备和应用是一项长期的工作，有限元法作为一种宏观尺度的数值模拟方法，目前主要用于化学凝胶的研究，而很少用于物理凝胶的研究，其原因可能是：与化学凝胶相比，物理凝胶的成胶过程更为复杂，这是由于体系组成和物理相互作用的复杂性更大。一个物理凝胶化过程可能涉及多种相互作用和步骤，在不同的条件下，其凝胶化机理往往不同，目前对多种物理凝胶化机理的认识还很不深入，唯象凝胶化动力学模型通常是非线性的，而物理凝胶化过程的动力学模型又往往是非线性的。对上述影响因素的数值模拟研究表明，由于材料的微观、介观结构与宏观性能之间的关系，宏观尺度下的凝胶化过程是一项困难的工作，为了优化原料配方和物理凝胶化条件，进而获得符合要求的聚合物物理凝胶，采用有限元方法研究了凝胶化过程中温度场的空间分布和时间演化特征，在计算温度场的基础上，根据一级凝胶化动力学方程对凝胶体系的复剪切模量场进行了数值模拟，讨论了原料浓度、样品尺寸和冷却方式对物理凝胶化的影响。凝乳酶酪蛋白凝胶是一种颗粒凝胶，其形成受包括空间水合力和货车范德华力在内的物理力控制，其结构和性质受包括乳化盐、pH、热历史和剪切历史。凝胶网络的形成过程可以分为三个步骤，分子-颗粒-絮体-网络结构，酪蛋白絮体是一种分形结构，可以被结构单元部分填充凝絮体的分形维数和凝絮体颗粒的大小是决定凝絮体微观结构和流变性能的两个重要参数。实验结果表明，冷却方式通过影响凝絮体颗粒的大小而影响凝絮体的流变性能。凝乳酶酪蛋白凝絮体的形成是凝絮体形成的第一步，凝絮体的分形维数和凝絮体颗粒的大小是决定凝絮体微观结构和流变性能的两个重要参数。研究了三个实例，实例1和实例2的原料浓度均为18%，但前者的样品量是后者的5倍;实例3的原料浓度为25%，样品量与实例1相同。凝胶带划分为136个单元163个节点，原型带划分为28个单元40个节点;在凝胶区选取了4个具有代表性的节点，系统初始温度设定为80℃，冷却方式包括（1）环境温度从80℃降至5℃，冷却速率分别为0.5K/min、0.25K/min，模拟结果表明：（1）表面附近的温度下降速率大于中心附近的温度下降速率，凝胶化首先发生在表面;由于无限复数剪切模量由预胶凝相的冷却速率决定，温度场的不对称分布导致了凝胶力学性能场的不对称分布，并且随着环境冷却速率和样品尺寸的增大，这种不对称性变得更加明显。
A physical gel is a three-dimensional network of polymer chains cross-linked by physical junctions,and it can arise either as the result of a phase transition or through some specific molecular association or as a result of entanglements.Due to the complexity of the inner structure and gelation mechanism,it is difficult to obtain purposefully polymer physical gel materials with specific constitution and properties.Thus,the controllable preparation and application of polymer physical gels are a long-term work.As a numerical simulation method in macro-scale the finite element method has been used to the study of chemical gelation,but rarely to physical gelation presently.The reasons may be:compared with chemical gels the gelation process of physical gels is more complex resulting from the bigger complexity of the system composition and physical interactions.Several kinds of interactions and steps may be involved in one physical gelation process.And under different conditions the gelation mechanisms are frequently different.By now the understanding of the gelation mechanisms of many kinds of physical gels is far from being intensive; the phenomenological gelation kinetics models are usually nonlinear; the theory describing the relation between the microscopic and mesoscopic structure and the macroscopic properties is still immature.For all the factors mentioned above the numerical simulation of physical gelation process in macro-scale comes to be a difficult work.In order to optimize raw material formulation and physical gelation conditions,and then obtain polymer physical gels according to requirements,the spatial distribution and time evolution characteristics of the temperature field during the gelation process were studied via the finite element method.On the basis of the temperature field calculated,the complex shear modulus field of gel system was numerically simulated according to a first-order gelation kinetic equation,and the effects of raw material concentration,sample size and cooling modes on the physical gelation were discussed.Rennet casein gel is a kind of particle gel,whose formation is controlled by physical forces including steric-hydration force and van der Waals force,and whose structure and properties are influenced by many parameters including emulsifying salts,pH,thermal and shear history.The formation process of the gel network can be divided into three steps,molecules-particles-flocs-network.Casein flocs are fractal objects that can be partially filled by the constructing units (particles).The fractal dimension of the flocs and the size of the particles are two important parameters determining the gel microscopic structure and rheological properties.Experimental results show that cooling modes will influence the rheological properties of the gel through influencing the size of the particles.The formation of rennet casein gels is a first-order kinetics process.Three examples were studied.The raw material concentrations of example 1 and example 2 are both 18%,but the sample size of the former is five times as that of the latter; the raw material concentration of example 3 is 25%,and the sample size is same with example 1. The gel zone is divided into 136 unites and 163 nodes; the prototype zone is divided into 28 unites and 40 nodes; four representative nodes were selected in the gel zone,and were emphatically studied.The initial system temperature is set at 80℃,and cooling modes include (1) environmental temperature declines from 80℃ to 5℃,and cooling rates are respectively 0.5 K/min,0.25 K/min,0.1 K/min,0.05 K/min,0.025 K/min; (2) environmental temperature maintains 5℃.The simulated results show that (1) the rate of temperature decrease near the surface is larger than that near the center,the gelation occurs on the surface first;as the infinite complex shear modulus is determined by the cooling rate of the pre-gelation phase,the value of the complex shear modulus of the interior exceeds that of the exterior finally.The asymmetric distribution of temperature field results in the asymmetric distribution of mechanical property field of the gel.And with the increase of environmental cooling rate and sample size the asymmetry becomes more obvious.