Small-angle neutron scattering studies on the phase behavior of binary polymer blends driven by photoisomerization
Small-angle neutron scattering studies on the phase behavior of binary polymer blends driven by photoisomerization
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光致异构驱动二元聚合物共混物相行为的小角中子散射研究
作者：
O. Urakawa;O. Yano;Q. Tran;A. Nakatani;C. Han
通过化学合成改变聚合物组分的立构含量，在许多聚合物共混物中研究了立构规整度 1-4 和微观结构 5、6 对混溶性的影响。改变聚合物链单体重复单元上的取代基也可以改变聚合物共混物的混溶性。如果能够通过某种与化学合成互补的方法连续改变表达聚合物链微观结构的参数，将更方便地控制混溶性。十年前，Irie和Iga证明，反式二苯乙烯标记的聚苯乙烯（PSS）和聚（乙烯基甲基醚）（PVME）混合物的较低临界溶解温度（LCST）可以通过紫外（UV）光照射来降低。 7 最近，我们使用 PSS/PVME 混合物作为模型系统，通过实验验证了对经历可逆反应的聚合物共混物中软模抑制的预测。 8 在这些实验中还发现，PSS/PVME 共混物因 313 和 365 nm 紫外光照射而在热力学上不稳定。这些共混物的优点是，在紫外光照射下，聚合物链的微观结构可以通过反式-顺式构象转变不断改变。由于二苯乙烯的顺式构象具有螺旋桨状构象，比平面反式构象大得多，因此预计共混物中PVME和PSS链之间的堆积在光异构化时变得低效，导致混溶性降低。 7-9 导致共混物热力学不稳定的另一个原因可能源于两种形式的二苯乙烯和 PVME 链段之间由于分子偶极矩的巨大变化而产生的不同相互作用。尽管已经证明 PS/PVME 共混物的二元相互作用参数随着压力的增加而降低，10, 11 对于 PSS/PVME 共混物中二苯乙烯反式顺式光异构化伴随的热力学不稳定，既没有直接证据也没有明显的解释。在这项工作中，我们通过使用小角中子散射（SANS）实验直接量化了辐照下PVME和二苯乙烯标记的氘代PSS链之间相互作用的变化。本工作中使用的样品是用反式二苯乙烯标记的氘化聚苯乙烯（PSD）（PSDS，Mw）3.3×105 g/mol，Mw/Mn）2.2）和聚（乙烯基甲基醚）（PVME，Mw）9.6×10 4 g/mol，Mw/Mn）1.6，购自Scientific Polymer Products Inc.12）。通过将反式 4-羟基二苯乙烯的钾盐（Lancaster，在甲苯中重结晶两次）与含有约 100 的 PSD 偶联，将二苯乙烯标记在 PSD 链上。通过苯乙烯-d8（剑桥同位素实验室）和（氯甲基）苯乙烯（CMS，东京化成株式会社）的自由基共聚，将 12.2 mol% 的氯甲基随机引入到 PSD 链中。 13 PSDS 的化学结构如图 1 所示。结合元素分析和紫外-可见光谱测定，氘代苯乙烯与残留（未反应）CMS 与标记单体的摩尔比分别为 0.877: 0.084: 0.039。重量分数 PSDS/PVME (20/80) 用作本研究中所有共混物的固定组成。 SANS 测量是在 NIST 中子研究中心的冷中子研究设施进行的。数据在 8 m SANS 仪器 (NG1) 上收集，中子波长 λ) 9.0 Å，提供的 q 范围为 0.008 Å-1< q< 0.08 Å-1，其中 q≠(4π/λ) sin (θ/2)，θ 是散射角。 SANS 的示例...
The effects of tacticity1-4 and microstructure5, 6 on miscibility have been examined in a number of polymer blends by varying the stereocontent of the polymer components through chemical synthesis. Changing the substituent groups on the monomer repeat units of polymer chains can also modify the miscibility of polymer blends. It would be more convenient for the miscibility control if the parameters expressing the microstructure of polymer chains could be varied continuously by some method complementary to the chemical synthesis. A decade ago, Irie and Iga demonstrated that the lower critical solution temperature (LCST) of the mixture of trans-stilbene-labeled polystyrene (PSS) and poly (vinyl methyl ether)(PVME) could be lowered by irradiation with ultraviolet (UV) light. 7 Recently, we have used PSS/PVME mixtures as a model system to experimentally verify the prediction for the soft-mode suppression in polymer blends undergoing reversible reactions. 8 It was also found in these experiments that the PSS/PVME blend was thermodynamically destabilized by irradiation with both 313 and 365 nm UV light. The advantage of these blends is that the microstructure of polymer chains can be continuously altered via trans f cis conformational transitions upon irradiation with UV light. Since the cis-form of stilbene has a propellerlike conformation that is much bulkier than the planar trans-form, it is expected that the packing between PVME and PSS chains in the blend becomes inefficient upon photoisomerization, leading to a decrease in miscibility. 7-9 Another reason responsible for this thermodynamical destabilization of the blend may originate from the different interactions between the two forms of stilbene and PVME segments arising from the large change in dipole moment of the molecule. Though it has been demonstrated that the binary interaction parameter,, of PS/PVME blends decreases with increasing pressure, 10, 11 there is neither direct evidence nor obvious explanation for this thermodynamical destabilization accompanying the trans f cis photoisomerization of stilbene in PSS/PVME blends. In this work, we directly quantify the change of the interactions between PVME and stilbene-labeled deuterated PSS chains under irradiation by using smallangle neutron-scattering (SANS) experiments. Samples used in this work are deuterated polystyrene (PSD) labeled with trans-stilbene (PSDS, Mw) 3.3× 105 g/mol, Mw/Mn) 2.2) and poly (vinyl methyl ether)(PVME, Mw) 9.6× 10 4 g/mol, Mw/Mn) 1.6, purchased from Scientific Polymer Products Inc. 12). Stilbene was labeled on the PSD chains by coupling the potassium salt of trans-4-hydroxystilbene (Lancaster, recrystallized twice in toluene) to the PSD containing ca. 12.2 mol% of chloromethyl groups that are introduced randomly into the PSD chain via the radical copolymerization of styrene-d8 (Cambridge Isotope Laboratory) and (chloromethyl) styrene (CMS, Tokyo Kasei Co.). 13 The chemical structure of PSDS is shown schematically in Figure 1. The molar ratio of deuterated styrene versus residual (unreacted) CMS versus labeled monomer is 0.877: 0.084: 0.039, respectively, as determined by the combination of elemental analysis and UV-visible spectroscopy. The weight fraction PSDS/PVME (20/80) is used as a fixed composition for all the blends in this work. SANS measurements were carried out at the Cold Neutron Research Facility of the NIST Center for Neutron Research. Data were collected on the 8 m SANS instrument (NG1) with the neutron wavelength, λ) 9.0 Å, providing a q range of 0.008 Å-1< q< 0.08 Å-1 where q≡(4π/λ) sin (θ/2) and θ is the scattering angle. The samples for SANS …