Isotactic Polystyrene Single-stranded Oligonucleotide Chain And Multi-chain Aggregates Of Crystallization, Structure And Morphology,

The isotactic polystyrene (i-Ps) was chosen as a sample, and amorphous aggregates of single-, pauci- and multi-chain particles were prepared by spraying-freeze-drying (SFD) procedure from dilute solution of i-Ps in benzene with concentration ranging from 1.0×10^-3g·mL^-1 to 2.0×10^-5g·mL^-1. The morphology of i-Ps single-, pauci- and multi-chain particles was observed, and the particle dimension was calculated. The investigation of cold crystallization and crystal morphology was carried out for the samples prepared from solutions of various concentrations, and also linear growth rate of spherulites crystallized from the melt of aggregate was measured and the crystal structure was characterized. The influence of entanglements on the crystallization process, structure and morphology was revealed by a parallel study of the properties of these single-, pauci- and multi-chain particles with those of bulk sample,. According to the crystallization theory of macromolecules, the thermodynamics parameters and structure dimension were calculated and analyzed in order to get a better understanding of crystallization process of polymers on a molecular level.By means of TEM, we observed the morphologies of amorphous single-, pauci- and multi-chain aggregates prepared by spraying-freeze-drying from solutions with various concentrations. It was found that the i-Ps aggregates prepared from the concentration of 2.0×10^-5g·mL^-1 is composed of isolated globules and most of themhas a diameter about twice times of the 0²>^1/2 of i-Ps random coil under θ state.This leads to an convincible conclusion that isolated single-chain coil can be prepared by spraying-freeze-drying method from extremely dilute solution. Electron diffraction pattern of i-Ps single-chain globules showed that a certain short-range order is maintained during the freeze-drying process, and it is further confirmed by fourier transform infrared spectrograph (FTIR) spectrum of single-chain aggregates after separation of bands in the region 500-600cm^-1. The FTIR spectrum also showed that the packing of segments in the aggregates prepared by SFD method is in a more dilated state than that of bulk sample.The cold crystallization behaviors of the single-, pauci- and multi-chain aggregates were investigated by DSC. It was found that as the solution concentration decrease, the cold crystallization temperature of the samples prepared from solutions was greatly decreased. For the samples prepared from three solutions mentioned above, the cold crystallization temperature is about 46.3°C, 24.9°C or 14.7°C lower than that of bulk sample, respectively. This depression in cold crystallization temperatureindicates an acceleration of crystallization rate, which could be ascribed to fewer and even no interchain entanglements in i-Ps aggregates prepared from more dilute solution. On the other hand, the melting point of i-Ps crystals formed in aggregates decreases with decreasing solution concentration, most likely due to the more smaller crystals were obtained in the case of solution with more dilute solution. The sample was heated in DSC cell to the end point of melting peak, followed by quenching to room temperature, and this procedure was repeated several times. It was observed that with the increase of DSC scan times, the cold-crystallization peak and melting peak shift to the higher temperature. Clearly, this is because the increase of entanglement density. While the aggregates were heated at 243°C for 40min, the single- and pauci-chain coils interpenetrated each other by thermal diffusion to form an entangled network with the same entanglement density as the normal bulk, therefore, the recovery of crystallization behavior characterized by bulk i-PS was observed in DSC scan. On the basis of the experimental observation, it could be put forward that during the crystallization the macromolecules should overcome the retardation of entanglement to approach the growth surface by self-diffusion.The growth rate of i-Ps bulk and aggregates prepared from various solution concentrations were measured under polarized optical microscope (POM) equipped with a hot stage in the range of 165°C-200°C. Different simplifications and treatments of nucleation term and transport term were discussed based on the LH theory of Surface-Nucleation and Growth. It is found that the nucleation factor K_g and activation energy of transport B^* calculated according to the growth rate are consistent with the classic literature. For i-Ps aggregates prepared by SFD procedure, the prefactor Go and nucleation factor K_g were also obtained, and they are found to decrease with decreasing entanglements, but the transport factor B^* is showed to increase with decreasing entanglements. The physical meaning of K_g were carefully discussed based on the Edwards-Philips method, and it is found that with decreasing entanglements, tight folding is more preferential during the substrate completion in the sample prepared by SFD method, and the end surface become less rough. Thus, it is reasonable to assume that the depression in the congestion of segments at melt-crystal interfacial would lead to a smaller value of end surface free energy. A new series of values were attained for thermodynamics parameters G₀, K_g, B^* , σ_e and σ according to the measurement of growth rate of crystals formed in aggregates melt. A more satisfied coincidence was achieved for the crystal dimension along chain direction from both approaches: one obtained from (220) reflection in WAXD pattern and another calculated by using melting point and new thermodynamics parameter σ_e.To our knowledge, all these results mentioned here are firstly reported until now.The morphologies of spherulites crystallized from the melt of bulk sample and aggregates prepared from various concentrations were observed by POM, and all spherulites are optically positive. The morphologies of crystals grown from the amorphous state of single- and pauci-chain aggregates at 195°C were investigated by scanning electron microscopy (SEM), abundant leaflike crystals were observed, which have not seen before. Such kind of crystals can be ascribed to the coexistence of low index growth face and high index ones, and they may be formed owing to the lack of available stems. Spherulites grown from the melt of the particle aggregates was observed by SEM after treating with Permanganic etchant, and they are found to be of sheaflike. Observation on the fine texture of spherulites shows that the space-filling mechanism of spherulite growth changes from "branching" to "spawn". While the spherulites grown in single-chain sample is composed of untwisted and unbranched lamellae. These observations are reported for the first time and they indicate that entanglements play a very important role in morphology and growth mechanism of crystals. Meanwhile, the Compact Seaweed (CS) crystal was observed to coexist with sheaflike spherulites in the melt-crystallized single-chain samples, indicating a strong diffusion-controlled characteristic of crystal growth. Various morphologies of hexagonal lamellae aggregates were observed, such as regular, rounded, and polyhedral with a sheaflike interior fibrosity; their formation is dependent on the crystallization condition. Such kind of crystal structure and morphology is consistent with Bassett's observation. What's more, the screw growth of some hexagonal lamellae aggregates is observed at the center of hexagonal lamellae aggregates, and with development of the screw, the lamellae change its arrangement from "flat-on" to "edge-on". The central parts exhibit a negative birefringence while they were observed by using POM.