Preparation And Study Of Polyacrylate Transparent Viscoelastic Film For Flexible Display

Transparent Viscoelastic Film（TVF）is often used as a adhesive layer to enhance display contrast due to its excellent optical property and good pressure sensitive adhesion.However,with the development of consumer electronics industry,new display forms such as curved screens,foldable screens and scrolling screens have gradually emerged,which pose new requirements and challenges to the combination performance of TVF.Firstly,TVF in flexible display should bear maximum deformation（at least more than 600%）and disperse stress to create a neutral layer for protecting the fragile functional devices,which requires that TVF has a sufficiently low modulus and its modulus should remain stable over a wide range of application temperatures.Secondly,in addition to providing effective bonding as a adhesive layer,TVF should maintain a high recovery rate after repeated large deformation and recovery.The conventional TVF with high modulus is prone to cracking,breaking and buckling during bending process and is not competent for flexible display applications.Altering the glass transition temperature（T_g）of the copolymer is the most efficient strategy to control the low modulus of the acrylate TVF,which can be achieved by selecting the type and the ratio of unit component of the acrylate copolymer backbone.Changes in the composition of the acrylate copolymer will affect the modulus,adhesive force,deformation recovery rate and other properties of TVF,which are not easily combined and need to be precisely regulated to obtain a good overall performance of flexible TVF.This thesis firstly investigated the biopolymer TVF containing carboxyl groups,and mainly investigated the effects of carboxyl polar component content and side chain carboxyl flexibility on the overall performance of TVF（rheological properties,adhesive properties,stress relaxation and deformation recovery,etc）.Then this thesis investigated the biopolymer TVF containing hydroxyl groups and mainly investigated the effects of flexible primary monomer structure and side chain hydroxyl flexibility on the overall performance of TVF under the same reaction conditions.Based on the study of the TVF containing hydroxyl groups,in order to improve the mechanical capability of isooctyl acrylate（EHA）/hydroxybutyl acrylate（HBA）and broaden the application range,acrylic acid（AA）,β-carboxyethyl acrylate（CEA）,acrylamide（AM）,isobneol acrylate（IBOA）were introduced as the third component respectively and the effects of the third component on the overall performance of TVF was investigated under the same reaction conditions.The main research contents and results include:（1）Two types of copolymers containing carboxyl groups,EHA/AA and EHA/CEA,were synthesized by solution polymerization using EHA and two carboxylate monomers,CEA and AA,respectively,with a molar composition of n（EHA）/n（carboxylate monomer）=96/4,92/8,85/15.TVFs containing carboxyl groups were prepared by adding aziridine crosslinkers and curing at 80°C for 18h.The study found:The decrease of carboxyl polar component content significantly reduces T_g,storage modulus and low temperature hardening degree of TVF,but also reduces the adhesive property,1000%strain stress relaxation rate and the hysteresis area of 1000%strain continuous cyclic tensile of TVF.Compared with EHA/AA system,EHA/CEA system has lower T_g,G’,low temperature hardening,and significantly improves the adhesive performance.The recovery rate of the shear creep test of TVF containing carboxyl groups is controlled by viscoelastic response and grows first and then descends with the increase of carboxyl functional monomer.Of all TVFs containing carboxyl groups,P_E96CEA4S_0.10 and P_E92CEA8S_0.10 have good low temperature moduli（when the modulus reaches 100k Pa,the temperature is less than-10℃）;excellent adhesive performance（the peeling force is8.514N/25mm and 15.593N/25mm respectively）;excellent 1000%strain stress relaxation（stress relaxation rate>37%after two cycles）;excellent 1000%strain continuous cyclic tensile recovery rates（recovery rate>83%after nine cycles）;excellent shear creep recovery rates（83.95%and 89.15%respectively）;thermal stability over the application temperature range and excellent optical properties.（2）Four types of acrylate copolymers EHA/HEA,HA/HEA,EHA/HBA,HA/HBA were synthesized by solution polymerization using EHA and n-hexyl acrylate（HA）as flexible primary monomers and hydroxyethyl acrylate（HEA）and HBA as the hydroxyl functional monomers respectively.TVFs containing hydroxyl groups were prepared by adding isocyanate crosslinkers and curing at 80°C for 18h.The study found:compared with EHA system,the HA system has lower T_g and temperature hardening,a more stable G’over a wide temperature range,a slightly lower adhesive property and a better deformation recovery rate;the HBA system has the better flexibility and hysteresis retention than the HEA system.P_H85HBA15N_0.10 has an excellent low temperature modulus;a good adhesive property with a peel force of9.060N/25mm;an excellent 1000%strain continuous cyclic tensile recovery rate（>84%）and a high shear creep recovery rate（88.18%）;thermal stability over the application range and an excellent optical property.（3）AA,CEA,AM and IBOA were used as the third enhanced monomer（EM）respectively,and the terpolymer was synthesized with a molar ratio of EHA/HBA/EM of 84/14/2.Three-component TVFs were prepared by adding isocyanate crosslinkers and curing at 70°C for 16h.The study found:A little enhanced monomer will increase T_g of the three-component TVF,however the low temperature modulus hardly increases.Strongly polar monomers（AA,CEA,AM）have a greater impact on the overall performance of TVF than the non-polar monomer IBOA.The AA ternary system and the CEA ternary system undergo a self-cross-linking reaction at higher temperatures,which increases the deformation recovery rate of TVF while significantly reducing the adhesive properties of TVF.P_E84H14AM2N_0.10 has an excellent low temperature modulus and a good adhesive property（a peel force of 8.862N/25mm）;an excellent1000%strain continuous cyclic tensile recovery rate（>84%）and a high shear creep recovery rate（87.93%）;thermal stability over the application range and an excellent optical property.