Investigation Of Multi-Scale Properties Of Polyurethanes Containing Non-Planar Ring Structures

Compared to planar benzene ring,non-planar rings are usually alicyclic structure,lipid heterocyclic structure and some other structures.These structures share one common point that they possess several conformations,which are capable of reversibly transform from one to another.In this work,the influence of the first order structure of molecular chain on the secondary structure and the macroscopic properties of polyurethanes,containing non-planar ring in hard segments,was studied,and we discussed at some extent the reasons leading to the different shape recovery ability of polyurethane samples.Polyurethane composed of 1,4-Phenylene diisocyanate(PPDI)and hydroquinone bis(2-hydroxyethyl)ether(HQEE),as hard segment,and Poly(tetramethylene glycol)(PTMG),as soft segment,was synthesized as a reference.For the first type of samples,a series of polyurethanes was prepared by gradually substituting PPDI with isophorone diisocyanate(IPDI).For the second and third types of samples,two series of polyurethanes were prepared by gradually substituting HQEE with 1,4-cyclohexanedimethanol(CHDM)or 3,9-bis(1,1-dimethyl-2-hydroxyethyl)-2,4,8,10-tetraoxaspiro[5.5]undecane(SPG).On one hand,single molecule force spectrum(SMFS)was performed to measure the segment elasticity and Kuhn length.It was easy to find that after benzene ring was substituted,the change of Kuhn length was small.However,the influence on segmental elasticity was obvious.Besides,owing to the changing structure of the main chain,microphase separation and macroscopic mechanical performance were altered significantly.The correlation between the micro-scale structure with nano and macro-scale performances was observed.On the other hand,the stretching and recovery processes of IPDI samples with 500% strain were monitored by small angle X-ray scattering(SAXS)and In-situ infrared dichroism.Interestingly,polyurethane samples showed two completely different evolution processes of phase morphology and molecular chain orientation,with low and high non-planar ring content respectively.Based on the dimension of the hard phases,we proposed two morphological evolution modes to explain the different recoverability of these samples.In the first mode,the hard segments of polyurethanes with low non-planar ring content formed lamellar hard domains.With a large strain,lamellar hard domains failed to afford external stress and then broke,in the meantime,stress-induced nanofibrils formed,which leaded to the poor shape recovery ability.In the second mode for polyurethanes with more non-planar rings in hard segments,much better stability of hard phase during elongation was observed.It was likely that non-planar rings hindered the stacking of hard segments,and the hard segments of these samples could only form fibrillar hard domains.During the deformation,fibrillar hard domains were stable,and network underwent affine deformation,leading to the final excellent shape recovery ability.