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Polymer chain entanglement on the glass transition

Author: RongWeiRen
Tutor: BoHaiShan;FanZhongYong
School: Fudan University
Course: Materials Physics and Chemistry
Keywords: Atactic polystyrene Single-stranded Oligonucleotide chain Entanglements The glass transition Relaxation time
CLC: O631.3
Type: PhD thesis
Year: 2007
Downloads: 484
Quote: 0
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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 (Tg) 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 Tg 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 Tg depression is not related to entanglement reduction. On the other hand, the Tg 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 (Tg) 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 Tg of samples decreases linearly with decreasing logarithmic concentration of solutions; while the Tg 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 Tg 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, Cs, 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 Tg was observed.The Tg 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, Tg should shift to higher temperature, and finally, slowly approach the bulk Tg. Dense packing of segments during annealing may take place and also have some contribution to the Tg shifting to higher temperature. The thermal diffusion and the interpenetrating rate of chains would rapidly increase with increasing temperature, and thus the Tg 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 Tg of freeze-dried samples prepared from very dilute solution recover back very slowly to bulk Tg, and could not reach bulk Tg 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 (Me~2×104 g/mol). Such low molar mass sample has a bulk Tg of 101℃, while the freeze-dried samples prepared from various solutions have a lower Tg, 2~4℃lower than the bulk value. For freeze-dried a-PS samples with high molar mass, prepared from very dilute solution, the Tg depression can be as large as~23℃. Also, the Tg 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 Tg 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 Tg 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.

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CLC: > Mathematical sciences and chemical > Chemistry > Polymer chemistry ( polymer ) > Polymer physics and physical chemistry of polymers > The chemical nature of polymers
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