Field-Cycling Relaxometry as a Molecular Rheology Technique: Common Analysis of NMR, Shear Modulus and Dielectric Loss Data of Polymers vs Dendrimers

详细信息    查看全文

• 作者：M. Hofmann ; C. Gainaru ; B. Cetinkaya ; R. Valiullin ; N. Fatkullin ; E. A. R枚ssler
• 刊名：Macromolecules
• 出版年：2015
• 出版时间：October 27, 2015
• 年：2015
• 卷：48
• 期：20
• 页码：7521-7534
• 全文大小：785K
• ISSN：1520-5835

文摘

Linear poly(propylene glycol) (PPG) as well as a poly(propyleneimine) (PPI) dendrimer with different molar masses (M) are investigated by field-cycling (FC) ¹H NMR, shear rheology (G) and dielectric spectroscopy (DS). The results are compared in a reduced spectral density representation: the quantity R₁(蠅蟿_伪)/R₁^伪(0), where R₁(蠅蟿_伪) is the master curve of the frequency dependent spin鈥搇attice relaxation rate with 蟿_伪 denoting the local correlation time, is compared to the rescaled dynamic viscosity 畏鈥?蠅蟿_伪)/畏鈥?sub>伪(0). The quantities R₁^伪(0) and 畏鈥?sub>伪(0), respectively, are the zero-frequency limits of a simple liquid reference system. Analogously, the dielectric loss data can be included in the methodological comparison. This representation allows quantifying the sensitivity of each method with respect to the polymer-specific relaxation contribution. Introducing a 鈥渃umulative mode ratio鈥?F_i(M) for each technique i, which measures the zero-frequency plateau of the rescaled spectral density, characteristic power-law behavior F_i(M) 鈭?M^伪_i is revealed. In the case of PPG, F_NMR(M), F_G(M), and F_DS(M) essentially agree with predictions of the Rouse model yielding characteristic exponents 伪_i. The crossover to entanglement dynamics is identified by a change in 伪_i around M 鈮?10 kg/mol. The analysis is extended to the dendrimer which exhibits a relaxation behavior reminiscent of Rouse dynamics. Yet, clear evidence of entanglement is missing. The M-dependencies of the dendrimer diffusion coefficient D obtained by pulsed field-gradient NMR and the zero-shear viscosity are found to be D(M) 鈭?M^鈥?.6卤0.2 and 畏(M) 鈭?M^1.9卤0.2, respectively, in good agreement with our theoretical prediction 畏(M) 鈭?M^1/3 D^鈥?(M). The close correspondence of R₁(蠅蟿_伪) with 畏鈥?蠅蟿_伪) establishes FC NMR as a powerful tool of 鈥渕olecular rheology鈥?accessing the microscopic processes underlying macroscopic rheological behavior of complex fluids.