7 Polymer viscoelasticity.pdf

ChapterChapterChapter 777
77 PPolymerolymer ViscoelasticityViscoelasticity
聚合物粘弹性聚合物粘弹性
¾ The study of polymer viscoelasticity treats the
interrelationships among elasticity, flow, and
molecular motion.
¾ In reality, no liquid exhibits pure Newtonian
viscosity, and no solid exhibits pure elastic
behavior, although it is convenient to assume so
for some simple problems.
¾ Rather, all motion of real bodies includes some
elements of both flow and elasticity.
¾ Because of the long-chain nature of polymeric
materials, their viscoelastic e to
the forefront. This is especially true when the
times for molecular motion are of the same order
of magnitude as an imposed mechanical motion.
PhenomenaPhenomena ofof
ViscoelasticityViscoelasticity
粘弹性现象粘弹性现象
Phenomena of Viscoelasticity 粘弹性现象
Polymer
聚合物
Strain
Ideal elastic material
虎克弹性体
Ideal viscous material
牛顿流体
Time
聚合物的粘弹性是指聚合物的力学性能与时间和温度有强
烈的依赖性,使其同时具有粘性液体和弹性固体的行为
CreepCreep 蠕变蠕变
„ 蠕变指在一定的温度和较小的恒定外力下,材料的形变
随时间的增加而逐渐增大的现象
ε
ε1
ε2+ε3
ε
Creep 2 Retraction
ε1
ε3
0 t
Three kinds of deformation 三种形变
„ Hookian elastic deformation 普弹形变
应力
ε1
σ 0
ε1 =
E1
t t
杨氏模量 1 2 t
键长、键角等的运动,可瞬间恢复
Three kinds of deformation 三种形变
„ Rubber elastic deformation 高弹形变
„ Retarded elastic deformation 推迟弹性形变
′
σ 0 −t τ推迟时间
ε 2 = (1− e )
E2
ε
高弹模量 2
τ’分子链从一个松弛的平衡态
构象变到一个紧张的平衡态
构象所需的时间
t
1 t2 t
链段运动,可逐渐恢复
Three kinds of deformation 三种形变
„ Plastic deformation 塑性形变
„ Viscous flow 粘性流动
σ 0
ε 3 = t
ε2
η0
本体粘度
t
分子间滑移,不可恢复 1 t2 t
Three kinds of deformation 三种形变
σ 0 σ 0 −t τ′σ 0
ε= ε1 + ε 2 + ε3 = + (1− e )+ t
E1 E2 η0
T<Tg ε1
Tg<T<Tf ε1+ε2
T>Tf ε1+ε2+ε3
StressStress RelaxationRelaxation 应力松弛应力松弛
„ 应力松弛指在恒定的温度和形变下,为维持此形变所需
的应力随时间增加而逐渐衰减的现象
−t τ松弛时间
σ= σ 0e
σ
Crosslinked polymer
σ0
Linear polymer
0 t