高分子链缠结对玻璃化转变的影响

【作者】 戎维仁；

【导师】 卜海山； 范仲勇；

【作者基本信息】 复旦大学 ， 材料物理与化学， 2007， 博士

【摘要】 缠结对玻璃化转变的影响是高分子科学中的一个有趣的基本问题，迄今尚无定论。近年来，有许多关于聚合物受限状态的玻璃化转变的研究。冷冻升华试样、由稀溶液的沉淀试样以及高分子薄膜的研究，都观察到玻璃化转变温度（T_g）的降低。对此提出了不同的解释，包括缠结浓度降低、表面层分子运动能力增强、和薄膜密度降低等。但有一些研究者提出T_g的降低与缠结浓度降低无关。另一方面，冷冻升华试样以及高分子薄膜的研究也观察到T_g的增高。为阐明玻璃化转变中缠结的作用，本文对无规聚苯乙烯的冷冻升华试样的玻璃化转变进行了详细研究。由无规聚苯乙烯苯溶液制备冷冻升华试样，制备溶液的浓度范围为：1×10^-1～2×10^-5g／ml。用差示扫描量热仪研究冷冻升华试样的玻璃化转变行为。研究发现；存在一临界浓度C_g^*(4×10^-3g／ml)，当制备溶液浓度大于G_g^*时，T_g不随制备溶液浓度的增加而变化，且与本体的玻璃化转变温度相同；当制备溶液的浓度小于C_g^*时，T_g随制备溶液浓度的对数的降低而线性降低。制备溶液浓度在C_g^*以上，溶液中的大分子链如同熔体中的一样，仍旧相互贯穿和缠结；冷冻升华后得到链的缠结网络，因而其T_g与本体试样相同。制备溶液浓度在C_g^*以下，随着溶液浓度降低，更多的孤立单链、寡链线团存在于溶液中；冷冻升华后得到更多的单链、寡链粒子，伴随着链间、链内缠结浓度降低。当由低于动态接触浓度，Cs，的溶液制备冷冻升华试样时，单链粒子的比例大大增加了，使缠结浓度进一步降低。链间、链内缠结浓度的降低增加了分子运动、降低了玻璃化转变温度。前人没有认识到动态接触浓度的存在，他们用了较高浓度的溶液(1×10^-3g／ml)去制备试样，分子链没有很好分开。在这样的溶液中，缠结浓度的降低是有限的，所以观察到较小的玻璃化温度降低。玻璃化转变温度降低也可能因冷冻升华试样密度降低引起。在稀溶液中，高分子链线团采取伸展链构，经冷冻升华的过程后，线团塌缩成紧密小球。球的堆积和链段在球内的堆积都很松，导致较低的密度。低密度对玻璃化温度降低会有部分贡献。冷冻升华试样的凝聚态远离平衡态，在高于试样玻璃化温度以上温度热处理，就会向平衡态过渡。在热处理过程中，孤立的单链、寡链线团通过热扩散会逐渐贯穿、再缠结。随着热处理时间延长，链间、链内的缠结浓度增加，同时链段的分子运动受到愈来愈大的阻碍，所以T_g不断移向高温、最后缓慢地接近本体的玻璃化转变温度。热处理过程中，链段的堆砌密度增加，也会对T_g移向高温有部分贡献。随着热处理温度的提高，链的热扩散和贯穿速度增加，T_g恢复到本体玻璃化转变温度的速度加快。随着制备溶液浓度的降低，T_g恢复到本体玻璃化转变温度的速度降低，很长时间退火以后，由极稀溶液制的冷冻升华的T_g仍旧不能恢复到本体玻璃化转变温度。用分子量低于临界缠结分子量的a-PS做了一组平行的实验。低分子量的冷冻升华试样的玻璃化温度比本体试样低2～4℃，而高分子量的冷冻升华试样的玻璃化温度的下降能达到～23℃。热处理时，低分子量的冷冻升华试样的玻璃化温度很快达到本体试样的T_g。低分子量的冷冻升华试样的玻璃化温度的降低可以认为是由于密度的下降。FTIR测量证实了冷冻升华试样中的链段堆积很松。比较本体试样，与苯环相关的分子振动谱带1601，1493，1452和698 cm^-1吸收峰变窄、变高，特别是吸收峰698 cm^-1更为明显。随着制备溶液浓度的降低，这些谱带的线宽和吸收强度的变化更为明显。退火时，链段的堆砌密度逐渐增加，并最后恢复到本体的水平，冷冻升华试样和本体试样的FTIR的差异随之消失。与玻璃化温度恢复本体的T_g相比，链段的密堆砌是一相对快的过程，因为后者仅需要链段的局部分子运动。二维FTIR测量指出：单、寡粒子附聚体退火时，与苯环相关的分子振动强度的变化先于CH₂基团而发生。这是因为前者对链段的密堆积更为敏感。可以认为缠结浓度的增加对分子振动只有很小的影响，因为链上两个缠结点之间约有200个重复单元。缠结对分子振动的影响，如果有的话，主要和CH₂基团有关，而且发生在退火过程的后期阶段，因为链的贯穿和缠结是较慢的过程，需要很长的时间才能完成。NMR技术也被用来研究由1×10^-2 g／ml、1×10^-3 g／ml和1×10^-4 g／ml溶液制备的冷冻升华试样，研究发现1×10^-4 g／ml溶液制备的试样的缠结浓度大大地降低了，而由两个较浓溶液制备的试样未见缠结浓度的明显降低。这一结果与DSC和FTIR的研究结果是一致的。更多还原

【Abstract】 The influence of entanglements on glass transition is an interesting and fundamental issue of polymer science, but so far it is still an open question. In recent years, there have been many investigations of glass transition on the confined polymer systems. The depression of the glass transition temperature （T_g） has been observed for the freeze-dried samples and precipitated samples from dilute solution, and for polymeric thin films. Different explanations were proposed for the T_g decrease, including a decrease in the entanglement, the enhanced mobility of surface layers, and a decrease in film density etc. However, some investigators proposed that the T_g depression is not related to entanglement reduction. On the other hand, the T_g increase was also observed for the freeze-dried samples and for thin films. In order to clarify the role of the entanglements in glass transition, a detailed study was carried out on the glass transition of a-PS freeze-dried samples.Freeze-dried samples were prepared from dilute solutions of atactic polystyrene （a-Ps） in benzene in a concentration range from 1×10^-1 to 2×10^-5 g/ml, and their glass transition temperatures （T_g） were determined by differential scanning calorimetry （DSC） and Fourier transform infrared （FTIR）. It was found that below a critical concentration C*_g(4×10^-3 g/ml), the T_g of samples decreases linearly with decreasing logarithmic concentration of solutions; while the T_g is not changed above C*_g with increasing solution concentration and it is the same as the glass transition temperature of the bulk sample. Above C*_g, the macromolecular coils still interpenetrate and entangle each other in the solution as in the melt, thus a network of chains obtained by freeze-drying method has a T_g just same as the bulk sample. Below C*_g, more isolated single-, and few-chain coils would exist in the solution with decreasing solution concentration, and thus more single-and few-particles were obtained by freeze-drying method, accompan?ng a decrease of inter-and intra-chain entanglements. The ratio of single-chain particles in the freeze-dried samples should greatly increases while the solution concentration is less than dynamic contact concentration, C_s, thus leading to further decrease of entanglements. The decrease of entanglements would enhance the mobility of segments and consequently reduce glass transition temperature. Prior investigators did not recognize the existence of dynamic contact concentration, and they used solution with a higher concentration (1×10^-3 g/mL) to prepare freeze-dried samples. Coils could not be well separated in this solution and the decrease of entanglements was limited in the freeze-dried samples, and therefore, a smaller depression of T_g was observed.The T_g reduction could be also due to lower density of the freeze-dried samples. The coils would have an extended conformation in the dilute solution and collapse into compact globules after freeze-drying procedure. Loose packing of both globules in the freeze-dried sample and the segments within globules leads to a lower density, which would have a partial contribution to the depression of glass transition temperature.The condensed state of the freeze-dried samples is far from equilibrium state and will change towards equilibrium state as long as the annealing temperature is higher than the glass transition temperature of the samples. During annealing process, the isolated single-and few-chain coils would gradually interpenetrate and re-entangle through thermal diffusion. As the number of inter-and intra-chain entanglements increases with annealing time, the molecular motion of segments is increasingly hindered. Therefore, T_g should shift to higher temperature, and finally, slowly approach the bulk T_g. Dense packing of segments during annealing may take place and also have some contribution to the T_g shifting to higher temperature. The thermal diffusion and the interpenetrating rate of chains would rapidly increase with increasing temperature, and thus the T_g return to equilibrium state would be faster on annealing at higher temperature. With decreasing the solution concentration used for the preparation of freeze-dried samples, the T_g of freeze-dried samples prepared from very dilute solution recover back very slowly to bulk T_g, and could not reach bulk T_g after annealing for very long time.A parallel study was performed on the glass transition temperature of a-PS freeze-dried sample with molar mass of 1．32×10⁴ g/mol, which is lower than the entanglement molar mass （M_e～2×10⁴ g/mol）. Such low molar mass sample has a bulk T_g of 101℃, while the freeze-dried samples prepared from various solutions have a lower T_g, 2～4℃lower than the bulk value. For freeze-dried a-PS samples with high molar mass, prepared from very dilute solution, the T_g depression can be as large as～23℃. Also, the T_g of the freeze-dried samples with low molar mass is easy to attain the glass transition temperature of bulk sample. It could be assumed that the T_g depression is caused by the density reduction.By FTIR measurement, loose packing of segments in the freeze-dried samples was demonstrated. All absorption bands at 1601,1493,1452 and 698 cm^-1 related to the molecular vibration of phenyl rings exhibit sharper and higher peaks compared with bulk sample, especially for the band at 698 cm^-1 . Such change in line width and absorption intensity was more significant with decreasing the solution concentration. During annealing, the packing of segments in the samples gradually increases, and finally, approach to that of the bulk sample, diminishing the differences in FTIR spectra between freeze-dried samples and bulk sample. Comparing the recovery of T_g to bulk glass transition temperature, dense packing of the segments is a relatively fast process because the latter only needs local molecular motion of segments.2-dimensional FTIR indicated the occurrence of change in the molecular vibration intensity of phenyl rings prior to one of CH₂ groups on annealing samples. This is because the former is more sensitive to dense packing of segments. The increase of entanglement concentration on annealing is assumed to have minor effect on molecular vibration since there are about 200 repeat units between two entanglements on the backbone, and the effect of entanglements on molecular vibration, if any, would mainly relate to CH₂ groups. Moreover, annealing for long time is needed to cause the increase of entanglement concentration and thus the effect of entanglements on molecular vibration would appear at the latter stage of the annealing process.NMR technique was also used to investigate the entanglement of freeze-dried samples prepared from 1×10^-2 g/ml, 1×10^-3 g/ml and 1×10^-4 g/ml solutions. A great decrease of entanglement concentration was found only in the sample prepared from 1×10^-4 g/ml, but not in the samples prepared from other two more concentrate solutions. The observation is consistent with the results from DSC and FTIR investigations.更多还原