Research On The Molecular Structure And Rheological Properties Of Branched GAP

Glycidyl azide polymer（GAP）is an energetic prepolymer with a side chain containing an azide group（-N₃）and a main chain with a polyether structure.It has outstanding advantages such as high heat generation,large density（more than 40% higher than HTPB）,clean combustion gas without smoke and corrosiveness,low characteristic signal,low combustion temperature,and low sensitivity.It is one of the key materials for energetic,low sensitivity and low signal propellants and an important part of the ideal energetic binder for solid propellants.Compared with linear GAP with the same molecular mass,branched GAP has higher functionality,lower glass transition temperature,higher heat generation,and lower cost.However,the molecular structure of branched GAP is more complicated.In order to promote the use of branched GAP in solid propellants,the molecular structure and rheological properties of branched GAP are studied in this paper.First,the molecular mass and molecular mass distribution of branched GAP were detected by gel permeation chromatograph（GPC）combined with an eighteen-angle laser light scattering instrument（MALLS）.The relationship between molecular mass and outflow time and relationship between hydrodynamic volume and time were studied.The results show that the relative standard deviation（RSD）of number-average molecular mass was 2.1%、RSD of weight average molecular mass was 1.8% and RSD of dispersion was 3.8%.The method can distinguish the molecular mass of different batches of branched GAP.The weight average molecular mass of branched GAP is generally on the order of 105.Secondly,the degree of branching of branched GAP was characterized by GPC combined with MALLS,a differential refractometer,and a viscosity detector.The intrinsic viscosity method with a small molecular mass（weight average molecular mass is about 5000）linear GAP as a standard and the mean square radius of rotation method with a wide molecular mass distribution polystyrene with a weight average molecular mass of 2 million were established for determination of branching factors of branched GAP.Both methods can characterize the degree of branching of branched GAP,and the mean square radius of gyration of linear GAP with the same molecular mass will be slightly smaller than that of PS,so the branching factor calculated using PS as the linear standard is smaller than the actual value,that is,the degree of branching is larger.Finally,the rheological properties of branched GAP were characterized using an advanced rotational rheometer.The dynamic and steady rheological properties of branched GAP were characterized.The results show that the branched GAP has shear thinning behavior.The logarithm of the energy storage modulus and the logarithm of the loss modulus of the branched GAP show a linear relationship with the logarithm of the angular frequency respectiovely,but both deviate from the viscoelastic theory.The branched structure of the GAP affects the dependence of the modulus on theangular frequency.In the range of 0.1¹00rad/s,the loss modulus is an order of magnitude larger than the energy storage modulus,and the branched GAP still dominates with viscous behavior.The relationship between the zero shear viscosity and weight average molecular weight of branched GAP is η₀=4×10^-10M_w^2.4,and its regression coefficient is 0.976.The greater the zero-shear activation energy,the more sensitive the zero-shear visvosity is to temperature changes.The greater the degree od branching in branched GAP,the greater the zero-shear activation energy.Increasing molecular mass and degree of branching will increase the shear rate at which visvosity tends to be stable.