聚对苯二甲酰对苯二胺纤维的热老化行为与机理研究

在250～400℃空气中对自由状态下对位芳纶进行等温热老化处理,采用万能材料试验机、红外光谱法、广角X射线衍射法、声速法和特性黏数法表征了老化过程中力学性能和结构的变化.结果表明,在老化初期,由于分子链的解取向,强度随时间快速下降;随后的热分解使强度随老化时间继续降低,符合二级反应动力学模型,其热老化表观活化能为32.4kJ/mol.老化样品的强度随温度升高显著下降,但高于350℃时热交联反应变得明显,同时结晶度增大和结晶结构完善,使强度的损失速率减小.老化样品的模量随老化温度的升高而增大,低于350℃时,非晶态分子链的解取向占优,模量较未热老化样品低;升高至350℃时,结晶结构完善占优,表现在第二类晶格畸变参数降低、表观微晶尺寸增大,特别是微晶横向融合使a,b轴方向尺寸显著增大,模量明显高于未老化样品.

THERMAL AGEING BEHAVIOR OF POLY(p-PHENYLENE TEREPHTHALAMIDE) FIBER

Poly(p-phenylene terephthalamide) fibers were treated in the range of 250 ～ 400℃ under free-end in air.Their mechanical properties and structure parameters were characterized by mechanical testing,Fourier transfer infrared spectroscopy,wide angle X-ray diffraction,sonic velocimetry and viscocimetry.It was found that the tenacity of aged samples decreased rapidly at the early stage of ageing due to the disorientation of molecular chains.Then,thermal degradation made the tenacity to keep on decreasing with time,which was consistent with the second-order kinetics equation with apparent activation energy of 32.4 kJ /mol.With the increase of ageing temperature,an obvious decrease in tenacity was found.However,the tenacity decreasing rate slowed down above 350℃ ,which could be related to the occurrence of slight cross-linking,the increasing of crystallinity and the perfection of the crystallites.The modulus of aged sample was lower than that of untreated sample,since the disorientation of molecular chains in amorphous region was dominant below 350℃ .The modulus of aged sample increased above 350℃ because of more perfect crystallization structure,which was indicated by the smaller distortion of second kind lattice,gII,and larger value of a,b axis due to the combination of adjacent crystallites.