摘要
苯乙烯(St)和丙烯腈(AN)在聚丙二醇(PPG)介质中的接枝共聚合产物——接枝聚醚多元醇(POP),可用于制备高性能聚氨酯泡沫和弹性体。该聚合过程属于典型的分散聚合,始于均相体系,由于St-AN共聚物(SAN)与PPG较差的互溶性,均相体系很快变成两相体系,即连续相(PPG富相)和分散相(SAN富相)。相分离以后,反应过程涉及反应物质的两相分配和两个聚合场所共存,使得聚合过程变得非常复杂。本文通过实验和模型研究了聚合过程中单体的相平衡规律、聚合反应动力学和颗粒的生长规律。
相平衡实验研究获得了大量的相平衡数据。St-AN-SAN-PPG四元二相体系可以看成两个拟三元体系,即St-AN-SAN和St-AN-PPG。基于Flory-Huggins(F-H)高分子溶液理论,建立了一个拟三元两相体系的相平衡模型,用于描述St和AN在相间的分配规律。采用Hildebrand-Scatchard溶度参数和Hansen三维溶度参数估算了温度影响的F-H相互作用参数,并通过实验数据拟合未知模型参数。模型能够较好地描述St和AN的分配行为。大部分情况下,St和AN都趋向于PPG富相。‘'St/AN配比”明显影响它们在相间的分配行为,降低"St/AN比值”,可同时提高它们的分配系数(SAN富相浓度和PPG富相浓度之比)。St和AN的分配系数受"SAN/PPG配比”和SAN共聚物组成的影响较小,随温度的增加而缓慢增加。拟合得到了用于计算聚合过程中单体分配系数的经验式。
对间歇和半连续接枝共聚合过程,进行了深入的实验研究,发现：
1)随聚合反应的进行,主要聚合场所逐渐从连续相转移到分散相,导致了聚合速率和分子量(MW)的激增、接枝效率的降低和分子量分布(MWD)的加宽。单体浓度的增加、共单体中AN含量的增加和反应温度的提高均能加速主要聚合场所的转移。
2)间歇聚合过程中,单体转化率(X)表现出明显的“本体聚合”特征。将共单体的滴加设定为“饥饿状态”时,半连续聚合过程中聚合速率可分为低速、恒速和低速三个阶段,固含率的变化相应呈现出缓慢增加、快速线性增加和缓慢增加三个阶段。
3)间歇和半连续聚合过程中,共聚物组成主要受"St/AN进料比”影响,表观恒比点为St/AN=70/30wt/wt。在75/25～60/40的"St/AN比值”范围内,整个聚合过程中共聚物组成没有明显的漂移。
4)接枝率主要受"St/AN进料比”影响,半连续聚合过程初期的接枝率高于间歇聚合过程初期,所得颗粒粒径也较小,但是后期的接枝率(0.2～0.3)没有明显区别。
5)间歇聚合过程中,不同"St/AN进料比”(74/25-60/40wt/wt)下,分子量(MW)及其分布指数(PDI)没有明显区别。但是,当St/AN=60/40时,半连续聚合过程中所得的MW及PDI明显高于间歇聚合过程,尤其在低单体进料速率下,高固含率下发生了化学交联。当St/AN=70/30或75/25时,两种聚合过程所得的MW和PDI几乎没有区别。
利用矩函数法建立了描述间歇、半连续和连续接枝共聚合过程的非均相接枝共聚合数学模型,模型由初始的“自由基溶液聚合模型”和后期“自由基两相聚合模型”组成。相分离之后,单体在两相间的分配系数采用相平衡模型或相平衡模型导出的经验式估算,引发剂和CTA假设在两相间平均分配,聚合物链的两相分配采用临界链长假定处理。分散相中的链终止、链增长和链转移速率系数及引发效率从该相一形成就采用CCS-AK扩散模型估算。利用间歇聚合过程所得的动力学数据拟合了未知模型参数。
模型能够较好地描述间歇聚合动力学,包括St和AN的转化率、PPG和大单体转化率、共聚物组成、分子量及PDI、接枝效率和接枝比,以及聚合过程中反应场所的转移。拟合得到了St-AN、AN-St、St-大单体、大单体-St间、AN-大单体和大单体-AN间的竞聚率。分散相中存在明显的“凝胶效应”和“笼效应”。大部分情况下,半连续聚合模型能够较好地描述聚合过程中体系的固含率及产品的结构特征(共聚物组成、接枝效率、分子量和PDI),但是不能很好地模拟St/AN=60/40下的MW和PDI,特别是在低进料速率下。
通过连续聚合过程建模仿真研究了不同反应条件对体系固含率及产品结构特征的影响,以及反应条件扰动对连续聚合过程的影响。固含率随单体浓度的增加而快速增加,接枝率随着"St/AN进料比”的降低而快速增加,MW随着"St/AN进料比”和反应温度的降低而明显增加,共聚物组成只受"St/AN进料比”的影响。与间歇和半连续聚合过程相比,连续聚合过程所得的接枝效率、MW和PDI较高。连续聚合过程中,反应条件的扰动后,体系可以较快达到稳定。
颗粒的形成遵循“均相聚并成核机理”,齐聚物的吸附和接枝共聚物的缠结共同作用。实验数据和模型计算结果表明,间歇聚合过程中颗粒成核期约在X=0.01-0.05。大单体和链转移剂(CTA)的初始浓度,及"St/AN进料比”对体系的稳定、颗粒粒径及其分布(PSD)具有重要作用。当St/AN<80/20wt/wt时,容易制得光滑的、球形的和具有清晰界面的聚合物颗粒。随着[CTA]。和共单体中AN含量的增加,颗粒粒径降低,PSD变窄。实验表明,粒径∝[单体]0.58·[大单体]-0.65·[引发剂]-0.06；理论表明,粒径∝[单体]2/3·[大单体]-0.5·[引发剂]-1/12,理论指数值与之吻合较好。模型能够较好地描述了不同实验条件下颗粒粒径的增长规律。随着引发剂浓度和反应温度的增加,颗粒粒径缓慢降低。随着“单体/PPG进料比”和"St/AN进料比”的增加,颗粒粒径缓慢增加。
对于聚合过程中的PSD,半连续与间歇聚合过程间存在明显区别。当大单体与PPG一起加入到反应釜中,半连续聚合过程的PSD会演变为双峰分布而后者一直为单峰分布。当大单体与单体料液一起滴加到反应釜中,半连续聚合过程的颗粒增长规律与间歇聚合过程相似,一直为单峰分布。半连续聚合过程中,可以通过“调控St/AN进料比”和“调控大单体的加料方式”调控颗粒粒径及其分布。
Graft copolymerization of styrene (St) and acrylonitrile (AN) in the presence of polypropylene glycol (PPG) is used to prepare graft polyether polyol (POP), which is widely applied in manufacturing high-performance polyurethane foams and elastomers. The present system starts from a homogeneous system and becomes a two-phase system (a PPG-rich continuous phase and a SAN-rich disperse phase) soon after the beginning due to the limitted misicbility between St-AN (SAN) copolymer and PPG. After then, polymerization involves partition behaviors of reactants and two polymerization loci, which makes the polymerization process very complicated. This work is aimed to disclose the polymerization kinetics and the particle growth. Mathematical models are also developed to describe batch and semibatch processes, and to simulate continuous process as well.
Equilibrium data of St-AN-SAN-PPG quaternary system were measured. Phase equilibria of the quaternary two-phase systems, St-AN-SAN-PPG, can be treated as phase equilibria between two pseudo-ternary systems, i.e. St-AN-SAN and St-AN-PPG. A pseudo-ternary two phase equilibrium model based on Flory-Huggins (F-H) polymer solution theory is established to describe partition behaviors of St and AN. The temperature-depedent F-H interaction parameters are estimated from Hildebrand-Scatchard solubility parameters and Hansen three-dimensional solubility parameters. The estimated F-H interaction parameters are also fitted by phase equilibrium data. The calculated results agree well with the experimental data under various experimental conditions. Both monomers are prone to exist in PPG-rich phase in most cases. St/AN ratio has a significant role in their partition coefficients, which are defined as the ratio of concentrations in SAN-rich phase to that in PPG-rich phase. The increase of AN amount can increase their partition coefficients, but St has the inverse effect. SAN/PPG ratio and copolymer composition have little impact on partition coefficients of St and AN. Partition coefficients of St and AN are increased slowly with increasing temperature. Two empirical expressions are obtained to calculate partition coefficients of St and AN during polymerization.
Basing on the extensive research on graft copolymerization experiments of batch and semibath processes, the following conclusions have been drawn.
1) During the polymerization, the main polymerization locus shifts from the continuous process into the disperse phase, leading to sharp increase in polymerization rate and molecular weight (Mw), decrease in graft effciency and broaden in molecular weight distribution (MWD). Increase of monomer concentration, AN fraction in comonomer and reaction temperature can advance the shift of main polymerization locus into the disperse phase.
2) Conversion evolution in batch process is similar to that in typical bulk polymerization and gel effect ocurrs soon after the beginning. When the feeds rate of monomer are set to be at a starvation state, the polymerization rate in semi-batch process can be divided into three stages: slow, constant and slow, and the increase of solid content can be also divided into three stages: slow, constant and slow.
3) Copolymer composition is only affected by comonomer feed ratio. The apparent azeotropic point is found to be around St/AN=70/30wt/wt. When St/AN ratio changes from75/25to60/40, there is no clear copolymer composition drift.
4) Graft efficiency is mainly affect by comonomer ratio. The initial graft efficiency in semi-batch process is higher than that in batch process, so the initial particle size prepared in semi-batch process is smaller than that in batch process. There is no obvious difference between graft efficiency in the later polymerization (ca.0.2-0.3).
5) In batch process, average weight molecular weight (Mw) and its polydispersity index (PDI) have no difference at different St/AN ratios. However, Mw and PDI prepared in semi-batch process at St/AN=60/40wt/wt is much higher than those prepraed in batch process, and some cross-linked SAN copolymers are formed at higher solid content under such conditions. At St/AN=70/30or75/25, there is no difference for Mw and PDI obtained between batch and semi-batch process.
Heterogeneous free radical polymerization models consisting of initial free radical solution polymerization models and later free radical two-phase polymerization models are developed for batch, semi-batch and continuous graft polymerization using moment method. After phase separation, partition coefficients of monomers are estimated by the phase equilibrium model or the empirical expressions derived from the phase equilibrium model; initiator and CTA are assumed to partition evenly between each phase; partition behaviors of polymer chains are evaluated by the critical chain length assumption. The well-known CCS-AK diffusion controlled model is used to estimate the initiation efficiency, kinetic parameters of termination, propagation and transfer reaction in the disperse phase from its formation. The kinetics data of batch process is used to fit the unknown model parameters.
The model describes the polymerization kinetics of batch process well, including monomer conversion, PPG¯omer conversions, copolymer composition, weight average molecular weight, PDI, graft efficiency and graft ratio, and describes the shift of polymerization locus as well. The reactivity ratios of St-AN pair, AN-St pair, St-macromer pair, macromer-St, AN-macromer pair and macromer-AN pair are fitted. In the dispersed phase, the obvious "gel effect" and "cage effect" are found. In most cases, the mathematical model for semi-batch process can describe properties of products well, including:solid content, copolymer composition and graft efficiency, Mw and PDI except for Mw and PDI at St/AN=60/40wt/wt, especially at low feeding rate.
Mathematical model for continuous process is used to simulate the effects of reaction conditions on solid content and product structure properties, and to carry out the stability analysis. Solid content increases sharply with increasing monomer concentration. Graft efficiency increases sharply with decreasing St/AN ratio. Mw increases with decreasing St/AN ratio and reaction temperature. Copolymer composition is only affected by comonomer ratio. Graft efficiency as well as Mw and PDI simulated in continuous process is higher than that obtained in batch or semi-batch proces. The reactor can tolerate the disturbances and insure that the transients will move back to the original steady state.
Based on "homogeneous aggregation nucleation mechanism", particle formation is cocontributed by absoption of oligomer radicals and entanglement of graft copolymers. Experimental data and simulated results observe that the nucleation occurs at monomer conversion of0.01-0.05in batch process, after then particle number keeps constant. Initial concentrations of macromer and CTA, and St/AN ratio have important roles on particle stability and PSD. Smooth, spherical and well-defined particles can be prepared when St/AN ratio is<80/20wt/wt. Particle size decreases and PSD becomes narrower with increasing CTA concentration and the AN fraction in comonomers. In theory, d∝Wmt02/3·WMo-0.5·[I]0-1/12; actually, d∝Wmt00.58·WM0-0.65·[I]0-0.06.The exponent values of the experimental dependence on monomer intial mass concentration (Wmto), macromer initial mass concentration (WM0) and initiator initial molar concentration ([I]0) is in good accordance with the theoretical values. The calculated conversion-dependent particle sizes under various experimental conditions agree well with the experimental data. Particle size decreases slowly with increasing initiator concentration and reaction temperature. Particle size increases slowly with increasing monomer/PPG ratio and St/AN ratio.
PSD prepared via semi-batch process differs much from that prepared by batch process. When all macromer is added into reactor with PPG firstly, the PSD in semi-batch process becomes bimodal PSD, but the PSD remains monodisperse in batch process. When all macromer is added into reactor with monomer mixture continuously, particle variation in semi-batch process is similar to that in batch process, and keeps monodisperse during the whole polymerization process. In semi-batch graft copolymerization process, the following two methods can be used to controll particle size and PSD:optimizing St/AN ratio and optimizing addition manner of macromer.
引文
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