| Decades has saw persisting difficulties for the precise determination of theta point in polymer solutions, where the long-range interactions vanish, the net interactive forces are balanced within the chain segments, and the excluded volume effects are absent. At this unperturbed theta condition, the investigation of dynamic behaviors of polymer flexible chains has been an ever-lasting topic in the field of polymer physics both experimentally and theoretically.;For a flexible polymer chain in a dilute solution, its individual segments are agitated by the thermal energy (kBT) to undergo the Brownian motions so that their center of gravity diffuses and the segments move randomly and relatively to the center of gravity. These motions can be described by a linear combination of different normal modes with corresponding characteristic relaxation frequencies. Theory has shown that for monodispersed polymer flexible chains in solution, the structure probed by dynamic light scattering (DLS) is dependent on the difference between the observation length 1/q and the size R of macromolecules. In detail, when 1/q > R only the translational diffusion is detected, and there appears a single peak in the spectral distribution S(q,o) of the scattered light; when 1/q < R the internal motions are probed as well, and spectral distribution S(q,o) shows an extra second peak. The smaller value of 1/q, and the larger intensity contributions from internal motions.;In Chapter 1, the properties of polymer in dilute solution is introduced as well as theories and developments of polymer internal dynamics during the past decades are summarized. In Chapter 2, the basic theory of laser light scattering and instrumentation are discussed in details. In Chapter 3, we studied the dynamics of poly(methyl methacrylate) (PMMA) in acetonitrile (MeCN) dilute solution. The slow aggregation of PMMA linear chains facilitates the dynamic study of PMMA single chain in MeCN below its theta point. At 44.9 °C (theta) of PMMA in MeCN, the longest internal relaxation time tau 1 was determined to be 0.16 +/- 0.02 ms. The plateau value of the reduced first Cumulant O* was calculated as 0.047 +/- 0.001, smaller than the values based on the nondraining model at theta condition with Oseen preaveraging. The normalized AI/(AI+AD) as a function of visolution temperature T shows a local maximum, giving the corresponding T* = 44.9 °C, which is close to the literature values (theta = 44 °C and 45 °C).;In Chapter 4, we studied the dynamics of polyisobutylene (PIB) in isoamyl isovalerate (IAIV) dilute solution. At 27 °C (theta), the longest relaxation time tau1 was measured as 0.20 +/- 0.05 ms. The plateau value of the reduced first Cumulant O* was determined to be 0.057 +/- 0.002. The normalized AI/(AI+AD) versus T shows both a turning point and a local maximum, yielding the corresponding T* = 27 °C and 25.5 +/- 0.5 °C, respectively, very close to the literature values (theta = 22.1, 25.0, and 27.0 °C).;In Chapter 5, the classic system of polystyrene (PS) in cyclohexane was investigated. At 34.5 °C (theta), the longest internal relaxation time tau 1 was 1.5 +/- 0.1 ms. The plateau value of reduced first Cumulant O* was determined to be 0.052 +/- 0.001. The normalized AI/(AI+AD) versus T shows a local maximum, with corresponding T* = 34.5 +/- 0.5 °C, very close to the reported values (theta = 34.0, 34.4, 34.5, 34.8, 35.0, and 35.2 °C). Also, the internal diffusivity Dint shows a local minimum around theta point, in the opposite way as the behavior of relative internal contribution versus temperature.;In Chapter 6, the system of poly(vinyl acetate) (PVAC) in heptan-3-one (EBK) was examined. At 29.0 °C (theta), the longest relaxation time tau 1 and the plateau value of the reduced first Cumulant O* were determined to be 1.2 +/- 0.1 ms and 0.050 +/- 0.002, respectively. Again, the normalized AI/(AI+AD) versus T shows a local maximum, giving a corresponding T* = 29.0 +/- 0.5 °C, close to the literature values (theta = 29 °C).;In summary, the static properties and the internal dynamics of four different polymer samples (polyisobutylene, poly(methyl methacrylate), polystyrene and poly(vinyl acetate)) solutions in extremely dilute solution were systematically studied by laser light scattering (LLS). We focused on the temperature dependent relative intensity of the internal motions AI/(AI+AD). Our results reveal that the solution temperature dependent AI/(AI+AD) shows a maximum or turning point (T*) near the theta temperature (theta), confirming a previous hypothesis; namely, the thermal energy (kBT) can excite more internal motions when twobody and higher-order interactions diminish. (Abstract shortened by ProQuest.). |
