单宁聚氨酯土壤微生物降解研究

采用凝胶渗透色谱仪(GPC)、傅立叶变换红外光谱仪(FT-IR)和电子扫描显微镜(SEM)等实验手段,考察了单宁聚氨酯(WT-PU)在土壤微生物降解前后的化学结构与微观形态的变化,而作为比较用的模型化合物(TMP-PU)在同样的条件下几乎没有变化。结果表明单宁一这交联点在PU整体中以无规降解的方式优先降解,在整个降解过程中,不仅伴随着PU硬段的氢键减弱,软段的氢键也同样有相当的减弱。     

骨架可水解的具核超支化聚缩醛的合成与表征

通过AB2型聚合单体4-(2-羟基乙氧基)苯甲醛二甲缩醛与B2型核分子苯甲醛二甲缩醛的缩醛转移聚合反应,反应过程中不断排出低沸点的醇,合成了具核、骨架可水解的超支化聚缩醛(HBPAs).实验表明,HBPAs的分子量,多分散性和聚合度随着核比例的改变发生明显的变化.增加核比例,聚合物的分子量,多分散性和聚合度均降低.HBPAs在弱酸性条件下,骨架发生水解,生成4-(2-羟基乙氧基)-苯甲醛.研究发现,核比例对于聚合物降解速率有明显的影响,增加核比例,聚合物的降解速率加快.这表明,通过加入核分子,可以在一定程度上调控超支化聚缩醛的结构与性能.     

煤改性聚乙烯塑料的降解性能及其机理探讨

通过对力学性能、红外光谱、粘均分子量及扫描电镜的分析,研究了煤/聚乙烯塑料在室内加速老化实验中的降解性能,并运用降解机理对实验过程进行解析及验证。结果表明,煤降解剂引发的交联和降解反应控制了薄膜的强度,使其柔韧性一直降低,且在整个120h光照过程中,断裂伸长率一直呈下降趋势;72h前是聚合物的氧化诱导期及衰变期,之后进入完全降解期。煤在改性塑料光照过程中引发自由基反应,引入羰基,导致聚乙烯大分子断链降解;共聚物的粘均分子量整体呈降低趋势,说明光照促进聚合物的降解,降解和交联交替控制着反应。煤/聚乙烯的光降解过程遵循链引发、链增长、链终止反应机理,在煤大分子光催化作用下,改变了聚乙烯常规光降解过程,加速了聚乙烯大分子断链和分子量降低。运用煤/聚乙烯塑料降解机理,能够解释样品力学性能变化、化学结构中羰基指数变化、分子量降低及降解过程中的现象和规律。     

生物降解高分子--聚己内酯/聚氧乙烯/聚丙交酯三元共聚物水解行为的研究

以异辛酸亚锡为催化剂,通过聚乙二醇醚(PEG)引发ε-己内酯和L-丙交酯开环聚合,制备了PCL/PEO/PLA三元共聚物.研究了聚合物在pH7.4磷酸缓冲溶液、37℃条件下的体外降解行为.采用GPC、1H-NMR、DSC和XRD技术研究了聚合物在水解降解过程中分子量、分子量分布、组成、吸水率、结晶性等的变化.结果表明共聚物的吸水率随聚醚组分含量而增大;随水解材料的失重率增大,聚醚组分含量下降程度也加大.此外研究还表明:聚合物中丙交酯组分含量高时,聚合物的结晶结构主要由PLLA形成.由于聚合物的水解降解首先发生在无定形区和结晶区边缘,随着共聚物的降解、结晶性的PLLA低聚物的生成,导致了共聚物的分子量呈双峰分布.     

聚合物分子量的教学思考

聚合物的结构控制是高分子化学的主要研究课题和关键教学课题,而分子量的控制是其重要内容。聚合物的分子量以平均值来表示,与聚合度直接关联。数均聚合度有其基本的数学定义式,根据聚合方式的不同,聚合反应的数均聚合度采取了不同的数学处理方式,其根源在于聚合反应的本质,其结果为数均聚合度的具体表达式;后者表现出不同聚合反应的分子量变化的相应特征,同时具有确定的适用范围。     

生物降解高分子——聚己内酯/聚氧乙烯/聚丙交酯三元共聚物水解行为的研究

以异辛酸亚锡为催化剂 ,通过聚乙二醇醚 (PEG)引发ε 己内酯和L 丙交酯开环聚合 ,制备了PCL/PEO/PLA三元共聚物 .研究了聚合物在 pH7 4磷酸缓冲溶液、37℃条件下的体外降解行为 .采用GPC、1H NMR、DSC和XRD技术研究了聚合物在水解降解过程中分子量、分子量分布、组成、吸水率、结晶性等的变化 .结果表明共聚物的吸水率随聚醚组分含量而增大 ;随水解材料的失重率增大 ,聚醚组分含量下降程度也加大 .此外研究还表明 :聚合物中丙交酯组分含量高时 ,聚合物的结晶结构主要由PLLA形成 .由于聚合物的水解降解首先发生在无定形区和结晶区边缘 ,随着共聚物的降解、结晶性的PLLA低聚物的生成 ,导致了共聚物的分子量呈双峰分布     

氧气在聚丙烯内吸附和扩散的分子模拟

采用巨正则Monte Carlo和分子动力学模拟相结合的方法研究了氧气在不同聚合度的聚丙烯内的吸附和扩散. 模拟结果表明, 随聚丙烯聚合度的增加, 聚丙烯对氧气的吸附量逐渐增加, 而氧气在聚丙烯内的扩散系数减小; 当聚合度增大到一定程度时, 吸附量和扩散系数都趋于一稳定值. 随温度的升高, 氧气在聚丙烯内的吸附量减少, 而扩散系数增大. 本文还应用自由体积理论探讨了氧气在聚合物内扩散的机理, 发现氧气在聚丙烯内以空穴形式存在的自由体积之间扩散, 即氧气先在一个空穴内不停振动, 然后通过聚丙烯链段运动形成的通道跳跃到下一个空穴来完成扩散. 结果表明, 较高聚合度的聚合物材料在常温及低温下使用对于其在食品包装材料中的应用是有利的, 这为食品包装材料行业相关产品的应用开发提供了一定的指导和依据.     

联吡啶铁/H2O2体系在可见光下降解芥子气模拟剂2-CEES

采用联吡啶铁/H2O2体系在可见光照射条件下(λ>450nm)降解芥子气模拟剂2-氯乙基乙基硫醚(2-CEES).考察了不同反应条件下的降解效果,通过EPR分析确定了反应过程中产生的高活性物种,利用GC-MS和NMR等方法分析了反应的产物,跟踪了反应过程中TOC的变化,并根据结果对2-CEES光催化降解的反应机理进行了探讨.     

钛电极苯酚降解反应动力学的Monte Carlo模拟

应用Monte Carlo方法,结合化学反应随机过程理论,模拟钛电极上苯酚电催化氧化反应机理,研究苯酚降解时间、初始浓度、电极的催化性能及反应中间产物对降解过程的影响.结果表明:苯酚浓度随时间变化逐渐降低并渐趋稳定,苯酚初始浓度较高时降解速率快,具有较高催化性能的电极对降解第1步反应的影响较大,可有效减少中间产物停留时间,但控制第1步反应则对降解过程尤其是第2步反应的产物有明显影响,而控制第2步反应对降解过程影响很小.     

通过熔体自由基反应调控聚丙烯体系的链结构和相结构

熔融反应加工是聚合物改性和制备聚合物纳米复合材料的重要途径之一.在此过程中,多数加成聚合物由于受到热、剪切或引发剂作用,通常可原位形成大分子自由基反应中间体.我们系统地研究了如何利用这类大分子自由基调控聚合物分子链的拓扑结构和聚合物纳米复合体系的相结构与界面.然而,某些聚合物大分子自由基,如聚丙烯(PP),受其分子链化学结构决定,在熔融反应条件下非常易于发生降解.研究发现,将可控自由基聚合中调控自由基反应活性的方法应用在熔融反应过程中可以显著抑制PP的降解,促进主反应的发生,在制备长链支化聚合物、调控聚合物纳米复合材料的相结构方面发挥了重要作用.本文介绍了本研究组近几年来通过熔体自由基反应调控PP体系的链结构和相结构的相关研究工作,如实现PP的长链支化,制备高熔体强度PP;在制备PP/C60 、PP/碳纳米管(CNTs)纳米复合材料过程中,利用熔体界面区域所发生的自由基反应,提高了纳米粒子与PP的界面相互作用,改善了纳米粒子在PP中的分散状态等.     

Degradation of poly(ethylene oxide) by high-speed stirring

Degradation of poly(ethylene oxide) (PEO) by violent stirring was studied in various solvents. The chain scission or the limit of the degradation was measured, and the effects of solvents, polymer concentration, stirring speed and degree of polymerization (DP) were investigated. It was found that the number of bonds broken per polymer chain was independent of the concentration but increased with the stirring speed and with decrease in the DP. The rate was much affected by the solvent used, being larger in a poor solvent. It was also found that the rate could be represented either by Jellinek's or Ovenall's rate equations, which have been applied to the ultrasonic degradation of polymers in solution.     

Photooxidative degradation of cellulose: Reactions of the cellulosic free radicals with oxygen

Photooxidative degradation of cellulose resulted in decreases of degree of polymerization (DP) and α-cellulose content, concurrently producing chromophoric groups; namely, carbonyl, carboxyl, and hydroperoxide groups within the polymer. Electron spin resonance (ESR) studies revealed that cellulosic carbon free radicals readily reacted with oxygen molecules at 143–160 K to produce peroxy radicals, whereas cellulosic oxygen free radicals were inert toward oxygen molecules throughout the photooxygenation reactions. At 77 K it is feasible that only photoexcited oxygen molecules reacted with cellulosic carbon free radicals to produce peroxide radicals. These radicals were themselves stabilized at 273 K by abstraction of hydrogen atoms from cellulose to produce polymer hydroperoxides. Simultaneously, new radical sites, which exhibited three-line ESR spectra, were generated in cellulose.     

Kinetics of thermal depolymerization of trimethylsiloxy end-blocked polydimethylsiloxane and polydimethylsiloxane-N-phenylsilazane copolymer

The thermal degradation of Me₃SiO end-blocked polydimethylsiloxane (eb-PDMS) and polydimethylsiloxane-N-phenylsilazane (eb-PDMS–NPhSz) copolymer was studied. For both polymers relative degree of polymerization (DP /DP ₀) as a function of conversion (1 – W/W₀) data were obtained. For eb-PDMS the results were consistent with a mechanism involving a rate determining random siloxane bond cleavage initiation step followed by a rapid and complete depropagation of the active fragments evolving volatile cyclic oligomers. Rate constants (k) for initiation were obtained at four temperatures from plots of DP ^?1 vs. time. An Arrhenius activation energy of approximately 80 kcal/mol was determined and is consistent with a SiOSi scission transition state. The degradation of eb-PDMS–NPhSz appears to follow the same depolymerization process evolving cyclic oligomers. Although DP /DP ₀ vs. C data suggest a random cleavage–complete depolymerization mechanism, an Arrhenius plot suggests a more complex degradation mechanism. The role of impurities as degradation catalysts is discussed.     

Novel theoretical and computer-assisted modeling of isothermal and non-isothermal depolymerization kinetics

A novel kinetic model accounting for the observed asymptotic approach of the degree of polymerization (DP) to a limiting value significantly greater than unity on prolonged degradation is derived and applied to the solid-state degradation of cellulose (Kraft paper) and poly(acrylic acid) (PAA) under isothermal and non-isothermal conditions. Experimental data were fitted using two iterative computer algorithms: one for isothermal DP data and the other for non-isothermal DP data obtained under a linear temperature ramp. The apparent activation energy for the solid-state recombination of chain radicals was found to be low in each case and was attributed to the proximity of free radicals being facilitated by restrictions imposed by the polymer matrix. The application of the model to non-isothermal DP yielded rate parameters that could be reconciled with those obtained from isothermal analyses, suggesting the novel approach has much merit for the future study of polymer degradation.     

Simulation of Degradation Processes. III. Testing of Some Relations for Random Chain Scission

Abstract

By means of the Monte Cario method a course of random chain scission was simulated and the following were decermined: the number-average moiecular weight and the degree of polymerization of the system, the weight-average moiecular weight and the degree of polymerization of the system, the number of molecules of degree of polymerization higher than an arbitrarily chosen value, and the number of molecules of a given degree of polymerization. Values obtained by simulation were compared with some analytical relations. Some hitherto unknown characteristics of the whole process have been established.     

On the degradation evolution equations of cellulose

Cellulose degradation is usually characterized in terms of either the chain scission number or the scission fraction of cellulose unit as a function of degree of polymerisation (DP) and cellulose degradation evolution equation is most commonly described by the well known Ekenstam equations. In this paper we show that cellulose degradation can be best characterized either in terms of the percentage DP loss or in terms of the percentage tensile strength (TS) loss. We present a new cellulose degradation evolution equation expressed in terms of the percentage DP loss and apply it for having a quantitative comparison with six sets of experimental data. We develop a new kinetic equation for the percentage TS loss of cellulose and test it with four sets of experimental data. It turns out that the proposed cellulose degradation evolution equations are able to explain the real experimental data of different cellulose materials carried out under a variety of experimental conditions, particularly the prolonged autocatalytic degradation in sealed vessels. We also develop a new method for predicting the degree of degradation of cellulose at ambient conditions by combining the master equation representing the kinetics of either percentage DP loss or percentage TS loss at the lowest experimental temperature with Arrhenius shift factor function.     

Simulation of degradation processes. II. Testing some relations for random crosslinking without cyclization

By use of a Monte Carlo method the course of crosslinking without cyclization was simulated. For the simulated case the following was determined: number-average molecular weight and degree of polymerization of the system; weight-average molecular weight and degree of polymerization of the system; number of molecules of a given degree of polymerization; quantity of gel; number-average degree of polymerization of sol and branching. The values obtained by simulation were compared with some equations published to date. It was shown that the dependence of weight-average polymerization degree and, consequently, of the weight-average molecular weight on time and the dependence of the number of molecules of a given polymerization degree on time is probably more complicated than hitherto supposed.     

Degradation of viscose fibers during acidic treatment

Novel developments in the fiber sector require additional investigation of their behaviour during the production process. In the core step of the viscose process, cellulose fibres are regenerated in an acidic spinning bath. To investigate the influence of hemicellulose content and temperature on the kinetics of fiber degradation, standard and hemicellose-enriched fibers were treated in the acidic standard spinning bath for time periods up to 292 h. Viscose samples of different hemicellulose content were prepared under standardized conditions and the never dried fibres were subjected to long term degradation in the standard spinning bath at 40, 50 and 60 °C. The changes in the degree of polymerization (DP), molar mass distribution, hemicellulose and the generation of the total organic carbon in the spinning bath were monitored. Further, the changes in crystallinity and the level-off-DP of the fibers were determined to improve the accuracy of the existing degradation model. Changes in morphology of the fibers were monitored by optical microscopy and scanning electron microscopy.     

无乳化剂乳液聚合制备高分子量聚乙烯醇

通过无乳化剂乳液聚合方法, 采用氧化还原引发体系制备了超高分子量的聚醋酸乙烯酯(PVAc), 继而醇解为超高分子量的聚乙烯醇(PVA). 研究了聚合温度、引发剂浓度、单体转化率对PVA的分子量和分子结构的影响. 探讨了线性高分子量PVA结构的控制方法. 结果表明, 利用无乳化剂乳液聚合可以实现在室温(14~20 ℃)制备出聚合度为9899的高分子量的PVA, 聚合过程对PVA的分子量和结构均有显著的影响. 在无乳化剂乳液聚合恒速聚合区得到的聚合物分子量较高, 分子量分布窄, 且结构比较规整, 而在加速区, PVAc的支化和交联现象显著, 最终会对PVA的线性程度产生很大影响. 因此, 可以通过聚合过程来控制PVA的分子量和链结构.     

Dynamic analysis of branching in radical polymerization

A method for the theoretical analysis of branching in radical polymerization is presented which includes the dynamics of the process. In particular, the method is applied to a polymerization that occurs by decomposition of initiator, propagation, termination by radical combination, and chain transfer with polymer. By a numerical solution of the kinetic equations (suitably transformed), the time dependence of the number-average degree of polymerization (DP), the weight-average DP, the mean number of branches, and the monomer conversion are obtained. The parameters of the process, that is the rate coefficients and initial concentrations, have the following effects: (1) An increase in the chain transfer coefficient increases the ratio of weight-average to number-average X_w/X_n and the mean number of branches X_b, but does not change the number-average X_n. (2) For a given value of the chain transfer coefficient, a change in the parameters of the process such that X_n increases, causes X_w/X_n and X_b to increase also. (3) Chain transfer with polymer seems to produce relatively few polymer molecules having many branches and a large number of smaller polymer molecules having no branches; consequently, the polymer size (or molecular weight) distribution broadens.