摘要
柔性聚合物链扩散穿过多孔膜的过程对凝胶色谱、超滤、可控释放和生物大分子的转移都具有十分重要的意义。本论文首先利用种子乳液聚合的方法合成了核不交联壳交联的核壳小球，并将其作为研究扩散的新型体系。通过激光光散射实时跟踪小球质量的变化，从而研究良溶剂中聚合物链从核内穿过多孔壳层的扩散过程。主要结果如下：
通过各种乳液聚合的方法(微乳液聚合、无皂乳液聚合和常规乳液聚合)，我们合成了粒径范围为0．05-1．0μm，窄分散的聚苯乙烯(PS)小球。分别通过激光光散射(LLS)和透射电子显微镜(TEM)表征了它们在溶液中和干态下的大小。两种方法都表明，所得的样品均为标准的球形且分散很窄，可以作为标准样品使用。同时，这也为我们合成窄分散的核壳小球打下了良好基础。
利用已合成的窄分散小球，通过在“饥饿状态”下缓慢滴加第二步单体的种子乳液聚合，我们将种子中的PS长链包裹在由交联PS链形成的薄层中。干燥后，将小球再分散/溶胀在无热(athermal)良溶剂中，核内的线型PS链就会穿过壳层中溶胀的多孔通道扩散出来。该扩散过程可利用激光光散射追踪。
首先，我们研究了相同交联度下(溶胀壳中的孔径相同)，不同壳层厚度对核内线型PS链透过溶胀多孔膜扩散的影响。结果表明，整个扩散可分为快慢不同的三个阶段。中间阶段是一个过渡阶段。三个不同的阶段代表了核内聚合物链溶液中相关长度(ξ)与多孔壳层中平均孔洞大小(d_(pore))之间的差异，即1)ξ＜d_(pore);2)R_g＞；ξ_c＞d_(pore);和3)ξ～R_g＞＞d_(pore)。我们首次观察到，当缠结的聚合物链在溶液中的相关长度小于孔径时，浓/亚浓溶液中的线型聚合物链在多孔膜中的扩散可快于其在稀溶液中的平动扩散。另外，我们还发现在第一阶段中，有效扩散系数基本相同，即壳层厚度对其没有影响。在第三阶段中，随着壳层厚度增加，扩散变慢。此时，核内的溶液已是稀溶液，相关长度远大于孔径，壳层厚度越厚，孔的长度越长，增加了对扩散的抑制。
我们还合成了壳层不同交联度但厚度相同的系列样品，并研究了交联度对核内线型聚合物链透过溶胀多孔膜扩散的影响。该情况下，扩散也分为三个不同的过程。在第一阶段中，有效扩散系数也大于其在稀溶液中的平动扩散系数。前述的解释在此也适用。在我们所研究的范围内，第一阶段的有效扩散系数与壳层的交联度无关。因为孔径均大于相关长度。将其与不同壳层厚度中的扩散对比，我们发现，在第一阶段中的有效扩散系数基本相同。这些事实说明，如果ξ_c＜d_(pore)，壳交联度和壳层厚度对扩散基本没有影响。因为，在第一阶段中，核内聚合物链处于浓/亚浓溶液中，相关长度只与浓度有关。在扩散的初始阶段，核中的浓度相近，故其有效扩散系数接近。当核中的溶液变为稀溶液后，壳层交联度越高，扩散越慢。此时，相关长度为链本身的大小，为一常数。孔径随交联度的增加变小。因此，核中的线型PS链更难进入壳层中的小孔，扩散变慢。
The diffusion of linear polymer chains through a porous membrane is important for gel permeation chromatography, ultrafiltration, controlled releasing and translocation of biological macromolecules. We have synthesized small spherical core-shell particles with a core made of linear non-cross-linked polystyrene (PS) chains and a cross-linked polystyrene shell by the seeded emulsion polymerization. These particles can be redispersed and swollen in an athemal solvent and used as novel system to study the diffusion of polymer chains through a porous membrane. Using a combination of static and dynamic laser light scattering to monitor the change of the average molar mass of these particles via the scattering intensity, we have examined the diffusion out of linear PS chains from the core through the porous shell. Our main findings are as follows.
Using a combination of different emulsion polymerization methods (micro-emulsion polymerization, surfactant-free emulsion polymerization and normal emulsion polymerization), we have synthesized different narrowly distributed PS particles with a diameter in the range 0.05-1.0μm. Their size distributions in water and in dry state have been characterized by laser light scattering (LLS) and transmission electron microscopy (TEM). The distributions of these PS particles are so narrow that they can be used as particle standards.
Using these narrowly distributed PS particles as the seeds (core), we were able to encapsulate long linear PS chains (core) inside a thin layer of cross-linked PS chains (shell) by the seeded emulsion polymerization in water under starved-condition. Such core-shell particles can be dried and redispersed/swollen in an athermal solvent (toluene). Using these small core-shell particles to study the diffusion, we gain two advantages: namely, 1) they have a huge surface area so that the diffusion can reach its equilibrium within a reasonable time, and 2) we can use LLS to follow the diffusion out of linear PS chains inside the core through the porous shell without any interference.
We have synthesized small spherical core-shell particles with different shell thicknesses, but the same core, to study the effect of shell thickness on the diffusion. Our results reveal that the diffusion has three stages, which can be attributed to a relative difference between the correlation length (ζ) of the polymer solution inside the core and the average pore size (d_(pore)) in the porous shell; namely, 1)ζ_c＜d_(pore); 2) R_g＞ζ_c＞d_(pore); and 3)ζ_c～R_g＞＞d_(pore). We have, for the first time, observed that the diffusion of linear chains from a concentrated/semidilute solution through a porous membrane is even faster than their translational diffusion in a dilute solution as long asζ_c＜d_(pore). In the third stage, the diffusion becomes slower as the shell thickness increases. This is because the polymer solution inside the core becomes dilute and the correlation length is equal to the chains size, a constant for a given solution. So that a thicker shell with longer pores slows down the diffusion.
On the other hand, we have synthesized small spherical core-shell particles with the same core and the shell thicknesses, but different cross-linking densities so that we can study the effect of cross-linking density on the diffusion. Our results show that the diffusion process also has three stages. Again, the diffusion of linear PS chains in the first stage is faster than their translational diffusion in a dilute solution. The previous discussion is also valid here. Moreover, in the first stage, the effective diffusion coefficients are similar in spite of different cross-linking densities. In comparison with the particles with different shell thicknesses, we find that the effective diffusion coefficient in the first stage is independent of both the shell thickness and the cross-liking density as long asζ_c＜d_(pore). In the third stage, the diffusion slows down as the cross-linking density increases. This is because the polymer solution inside the core becomes dilution and the correlation length remains a constant. A higher cross-linking density means small pores so that it is more difficult for linear PS chains to diffuse in.
引文
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