Vassiliadou Olga, Chrysostomou Varvara, Pispas Stergios, Klonos Panagiotis A, Kyritsis Apostolos
Department of Physics, National Technical University of Athens, Zografou Campus, 15780, Athens, Greece.
Theoretical and Physical Chemistry Institute, National Hellenic Research Foundation, 48 Vassileos Constantinou Avenue, 11635 Athens, Greece.
Soft Matter. 2021 Feb 15;17(5):1284-1298. doi: 10.1039/d0sm01666g.
In this article we present results on the glass transition, crystallization and molecular dynamics in relatively novel oligomers, oligo-ethylene glycol methacrylate (OEGMA), with short and long chains, as well as in the corresponding nanostructured comb-like polymers (POEGMA, short and long), the latter being prepared via the RAFT polymerization process. For the investigation we employed conventional and temperature modulated differential scanning calorimetry in combination with high resolving power dielectric spectroscopy techniques, broadband dielectric relaxation spectroscopy (BDS) and thermally stimulated depolarization currents (TSDC). Under ambient conditions short OEGMA (475 g mol-1, ∼4 nm in length) exhibits a remarkable low glass transition temperature, Tg, of -91 °C, crystallization temperature Tc = -24 °C and a significant crystalline fraction, CF, of ∼30%. When doubling the number of monomers (OEGMA-long, 950 g mol-1, chain length ∼8 nm) the Tg increases by about 20 K and CF increases to ∼53%, whereas, the Tc migrates to a room-like temperature of 19 °C. Upon formation of comb-like POEGMA structures the grafted OEGMA short chains, strikingly, are not able to crystallize, while in POEGMA-long the crystallization behaviour changes significantly as compared to OEGMA. Our results indicate that in the comb-like architecture the chain diffusion of the amorphous fractions is also strongly affected. The semicrystalline systems exhibit significant melt memory effects, this being stronger in the comb-like architecture. It is shown that these effects are related to the inter- and intra-chain interactions of the crystallizable chains. The dielectric techniques allowed the molecular dynamics mapping of these new systems from the linear oligomers to POEGMAs for the first time. BDS and TSDC detected various dynamics processes, in particular, the local polymer dynamics (γ process) to be sensitive to the Tg, local dynamics triggered in the hydrophilic chain segments by water traces (β), as well as the segmental dynamics (α) related to glass transition. Interestingly, both the short and long linear OEGMAs exhibit an additional relaxation process that resembles the Normal-Mode process appearing in polyethers. In the corresponding POEGMAs this process could not be resolved, this being an effect of the one-side grafted chain on the comb backbone. The revealed variations in molecular mobility and crystallization behavior suggest the potentially manipulable diffusion of small molecules throughout the polymer volume, via both the molecular architecture as well as the thermal treatment. This ability is extremely useful for these novel materials, envisaging their future applications in biomedicine (drug encapsulation).
在本文中，我们展示了关于相对新型的低聚物聚乙二醇甲基丙烯酸酯（OEGMA）的玻璃化转变、结晶和分子动力学的研究结果，该低聚物具有短链和长链，以及相应的纳米结构梳状聚合物（POEGMA，短链和长链），后者通过可逆加成-断裂链转移（RAFT）聚合工艺制备。为了进行研究，我们采用了常规和温度调制差示扫描量热法，并结合高分辨介电谱技术，即宽带介电弛豫谱（BDS）和热刺激去极化电流（TSDC）。在环境条件下，短链OEGMA（475 g/mol，长度约为4 nm）表现出显著的低玻璃化转变温度Tg为-91°C，结晶温度Tc = -24°C，以及约30%的显著结晶度CF。当单体数量翻倍时（长链OEGMA，950 g/mol，链长约8 nm），Tg升高约20 K，CF增加到约53%，而Tc迁移到类似室温的19°C。在形成梳状POEGMA结构后，引人注目的是，接枝的短链OEGMA无法结晶，而在长链POEGMA中，与OEGMA相比，结晶行为发生了显著变化。我们的结果表明，在梳状结构中，无定形部分的链扩散也受到强烈影响。半结晶体系表现出显著的熔体记忆效应，在梳状结构中这种效应更强。结果表明，这些效应与可结晶链的链间和链内相互作用有关。介电技术首次实现了从线性低聚物到POEGMA这些新体系的分子动力学映射。BDS和TSDC检测到了各种动力学过程，特别是对Tg敏感的局部聚合物动力学（γ过程）、由微量水引发的亲水性链段中的局部动力学（β），以及与玻璃化转变相关的链段动力学（α）。有趣的是，短链和长链线性OEGMA都表现出一个额外的弛豫过程，类似于聚醚中出现的正常模式过程。在相应的POEGMA中，这个过程无法分辨，这是一侧接枝链对梳状主链的影响。所揭示的分子迁移率和结晶行为的变化表明，通过分子结构以及热处理，小分子在整个聚合物体积内可能具有可控的扩散。这种能力对于这些新型材料极为有用，设想它们未来在生物医学（药物封装）中的应用。
