Solvent and Molecular Weight Dependence of Scaling Behavior in 6FDA-TFDB Polyimide Solution

Author:Chen Hong Xiang

Supervisor:ji xiang ling


Degree Year:2019





Polyimide(PI)is one kind of high-performance engineering plastics widely used in the aerospace,microelectronic,and fire-protection industries,etc.The previous studies on soluble PI solutions mostly focused on dilute solutions,however,the processing of polyimide products relies on concentrated solutions.The results on dilute solutions only provided less guidance for the concentrated PI solution properties.In recent years,our group had explored the solution properties of soluble 6FDA-TFDB PI in a wide concentration range.Some results are helpful to understand its scaling behavior,but a systematic study is still required on its solution properties.In this dissertation,a soluble 6FDA-TFDB PI was chosen,and the influence of solvent type and molecular weight on the scaling behavior of PI solutions was systematically investigated in a wide concentration range.The results in this study laid the foundation to regulate the properties PI solutions and final PI products.1.Various good/poor solvent pairs,including DMAc-H2O,THF-H2O,THF-formamide and THF-glycol,were chosen to explore their influence on the precipitation fractionation process of 6FDA-TFDB PI.GPC results indicated that the PI sample was fractionated in a descending order of molecular weight and the fraction had a narrower PDI compared with the pristine PI sample.By comparing the fractionation results of different systems,THF-H2O system was easy to operate and was the best solvent pair to fractionate 6FDA-TFDB PI sample.PI samples with various molecular weights and narrow PDI were easily and efficiently obtained through precipitation fractionation,which guaranteed the sample source for subsequent experiments.2.Four solvents,namely,N,N-dimethylacetamide(DMAc),N-methyl pyrrolidone(NMP),N,N-dimethylformamide(DMF),and dimethyl sulfoxide(DMSO),were selected to systematically investigate the influence of solubility parameter on the scaling behaviors of 6FDA-TFDB PI solutions.The scaling relationships between specific viscosity and concentration were established using rheology and applying the Zimm,Rouse-Zimm,and Doi-Edwards models.All scaling exponents were higher than theoretical predictions.In dilute solution,scaling exponents increased with increasing solubility parameter because of the different powers of solvents in screening weak interaction.In semidilute unentangled solution,scaling exponents decreased with solubility parameter because the scale of the effective dipole-dipole interaction space became small.In semidilute entangled solution,increased solvent solubility parameter resulted in stronger interaction between the chain segments and the "tube",which inhibited the reptation of PI chain from the "tube" and increased the scaling exponents.Moreover,the overlap concentration(C*)and entanglement concentration(Ce)decreased with increasing solubility parameter due to the increased polymer coil radius in solutions.3.The 6FDA-TFDB PI,with a relatively low molecular weight(Mw)of 88,000 g/mol,was dissolved in cyclohexanone(CYC).The scaling relationship between viscosity(η0-ηs)and volume fraction was in good agreement with the associative polymer theory proposed by Rubinstein and Semenov.Oscillatory rheological results indicated that the PI solution tends to become a gel with increased volume fraction.The synchrotron radiation small-angle X-ray scattering results implied the existence of dense aggregates in the concentrated PI/CYC solutions.Shear thickening and thinning behaviors were observed in the solutions,and the shear thickening behavior of PI solution had never been reported in literature before.The shear thickening behavior was due to the shear-induced increase in the number of elastically active chains.The shear thinning behavior in unentangled solution was due to shear-induced destruction of network.And this thinning behavior in entangled solution occurred not only from shear-induced destruction of network but also from the disentanglement of polymer chains during shear flow.Definitely,CYC as a solvent caused stronger dipole-dipole interaction in PI/CYC solutions compared with PI/DMF solutions.It demonstrated that variation of relatively weak interaction strength can transform PI from neutral polymer to associative polymer.4.Eight 6FDA-TFDB PI samples with absolute molecular weight ranging from 125,000 g/mol to 311,000 g/mol were obtained by precipitation fractionation.Rheological experiments were conducted to explore the effect of molecular weight on the associating behavior of PI/DMF solutions in a broad volume fraction,including abnormal steady shear flow,solution heterogeneity and scaling behavior.Abnormal flow behavior,i.e.,multi-region shear thinning and weak shear thickening were observed and these behaviors become less obvious with increasing volume fraction and molecular weight.The heterogeneity of PI/DMF solutions was examined by dynamic rheological test.By plotting ηsp versus φ/φη four concentration regions of I-IV were distinguished for all PI samples with various molecular weights.The scaling results in different concentration regions were in good agreement with the associative polymer theory proposed by Rubinstein and Semenov.The scaling exponents showed no molecular weight dependence in concentration region of I and II.In concentration regions of III and IV,the scaling exponents had little change when molecular weight was below 215 k,while scaling exponents increased with increasing molecular weight from 215 k to 311 k.The molecular weight of 215 k could be defined as a critical value,above which the association strength was largely enhanced with increasing molecular weight.5.One soluble 6FDA-TFDB PI sample with high absolute molecular weight of 390,000 g/mol was obtained by precipitation fractionation.The viscoelastic behaviors of PI in cyclohexanone(CYC)solution was investigated at various concentrations by conducting steady shear,dynamic shear,and stress relaxation rheological measurements.PI gel was able to form at a suitable concentration.The sol-gel transition was studied using scaling law and fractal analysis.The gel threshold cg and the critical relaxation exponent n were obtained based on the Winter and Chambon theory.The scaling relations η0∝ε-γ and Ge ∝εz were examined in the vicinity of gel concentration cg.The fractal dimension df of PI gel was determined through different methods.The influence of adding N,N-dimethylformamide(DMF)on the gelation behavior of PI solution was investigated,and found that the interchain dipole-dipole interactions between trifluoromethyl groups could be dissociated by adding a small fraction of DMF.By exploring the behavior of soluble 6FDA-TFDB polyimide in solutions can help us to gain deep insight into polyimide solution properties in a wide concentration range,and will guide the preparation of polyimide solutions for different processing,especially for the PI casting films and spinning fibers.