Chao Shen,Bailiang Xue,2,*,Danwei Xue,Rui Tang,Wei Zhao,Xinping Li

1.College of Bioresources Chemical and Materials Engineering,Shaanxi University of Science and Technology,Xi′an,Shaanxi Province,710021,China

2.Key Laboratory of Pulp and Paper Science&Technology of Ministry of Education,Qilu University of Technology(Shandong Academy of Sciences),Ji′nan,Shandong Provice,250353,China

Abstract:This study provides a comprehensive understanding of the polymeric properties of lignin-based non-isocyanate poly(iminehydroxyurethane)s(LNIPUs).The properties of the LNIPUs are affected by changes in the stoichiometric feed ratios of the bis(6-membered cyclic carbonate)(BCC)and levulinate enzymatic hydrolysis lignin(LEHL).The results showed that the LNIPUs exhibited a short relaxation time and excellent thermal repair and degradation properties.With a change in the LEHL content in the LNIPUs to 45.53%,a relaxation time of only 9 s was achieved,and the thermal repair rate of the scratches reached 93%.Furthermore,the tensile strength of the LNIPUs decreased with an increase in the LEHL content after two hot-pressing processes,while a higher than 75%tensile strength was maintained after the second hot-pressing treatment.The LNIPUs exhibited thermoresponsive shape memory property with deformation and shape fixing at 80℃.In addition,the as-synthesized LNIPUs were soluble in ethylene glycol in the absence of any organic solvents.This work demonstrates the synthesis of LNIPUs with self-healing,reprocessing,shape memory,and degradation properties.

Keywords:lignin;non-isocyanatepolyurethane;self-healing;reprocessing DOI:10.1213/j.issn.2096-2355.2021.03.001

1 Introduction

Lignin is the second most abundant natural polymer in plants and shows great potential as a desirable alternative raw material to fossil resources.However,less than 2%of the millions of tons of industrial lignin are used as natural stabilizers and surfactants for high value-added products[1],while the rest is disposed of or burned as low-value fuel.Lignin has been applied in a variety of products,including polyurethane(PU)[2],epoxy resin[3],and phenol resin[4];however,the use of lignin as a polyol in PU foam production is arguably the most advanced and studied application.

PUs are among the most widely used polymers worldwide,with an estimated annual production of 29 billion kg in 2020[5].However,isocyanates,the main component in the synthesis of PUs,are toxic,and the synthesis of isocyanates involves extremely hazardous substances,such as phosgene[6].There have been several attempts to prepare PUs without using isocyanates[7].Thus,the preparation of environmentally friendly non-isocyanate polyurethanes(NIPUs)as an alternative has attracted increasing attention owing to itsusein green materialsand procedures[8−12].

Currently,lignin or lignin-derived monomers used in the synthesis of NIPUs are produced through cyclic carbonate-functionalized lignin derivatives,which are mainly obtained via the glycidylation of lignin,followed by cycloaddition with CO2[13−15].However,this strategy requires highly toxic epichlorohydrin and high-pressure equipment to achieve the insertion of CO2.Furthermore,Kühnel et al[16]reported the preparation of cyclic carbonate-functionalized lignin through the oxyalkylation of lignin with glycerol carbonate to insert adjacent hydroxyl groups,and its subsequent reaction with dimethyl carbonate via a transesterification.However,this method leads to large amounts of coproducts associated with the oxyalkylation(CO2)and transesterification(MeOH)reactions.Huang et al[17]reported a facile strategy for the modification of lignin with levulinic acid via esterification.The levulinate enzymatic hydrolysis lignin(LEHL)could be reacted with diamines to produce polyimine thermosets. In our previous study[18], we first synthesized NH2-terminated polyhydroxyurethane(NPHU)prepolymers using bis(6-membered cyclic carbonate)(BCC)and diamines via a ring-openingreaction,and obtained thelignin-based nonisocyanate poly(imine-hydroxyurethane)s(LNIPUs)via a Schiff base reaction between the levulinate lignin derivatives and the NPHUs.The structural,mechanical,and thermal properties of the LNIPUs were comprehensively studied.

In this study,in addition to investigating the mechanical and thermal properties of the LNIPUs,the effects of varying the stoichiometric feed ratios of BCC and LEHL,self-healing,thermal repair,reprocessing,shape-memory capability,and degradation properties of LNIPUs were comprehensively investigated.These materials represent a significant step forward in the development of a sustainable approach to the synthesis of LNIPUs with self-healing,reprocessing,and degradation properties.

2 Experimental

2.1 Materials

Enzymatic hydrolysis lignin(EHL)was purchased from Longlive Biological Technology Co.,Ltd.(Shandong province, China). The chemical characteristics of lignin were reported in our previous study[19].Levulinic acid(LA),N,N'-dicyclohexylcarbodiimide (DCC), JEFFAMINE®D-400 polyoxypropylenediamine (D-400),4-dimethylaminopyridine(DMAP),and pentafluorobenzaldehyde were purchased from Aladdin (Shanghai,China). Di(trimethylopropane)(98%)was purchased from J&K Scientific(Beijing,China)and used as received.Diphenyl carbonate was purchased from Sinopharm(Beijing,China).All the reagents were of chemical grade.

2.2 Methods

2.2.1Preparation of the LNIPUs

The NPHU prepolymers were synthesized according to our previous study[18]by using different amounts ofBCC(0.9676,0.9192,0.8708,0.6774,and 0.4838 g)and D-400.LEHL was obtained via an esterification reaction between EHL and LA.A series of LNIPUs were prepared using different NPHU prepolymers and varying the LEHL content(0.07,0.15,0.89,and 1.47 g).By varying the molar ratios of the BCC cyclic carbonate groups and LEHL ketone groups(denoted as BLx:y,of whichx∶y=BCC:LEHL=100∶0,95∶5,90∶10,70∶30,and 50∶50,w/w),the weight percentages of LEHL in the LNIPUs were varied between 0,3.24%,6.42%,31.13%,and 45.53%to produce BL100:0, BL95:5, BL90:10, BL70:30, and BL50:50,respectively.

2.2.2Characterization

2.2.2.1Relaxation property

Stress relaxation analysis was performed on a DMA Q800 instrument(TA Instrument,US).The film sample(30 mm×5 mm×0.5 mm)with uniform thickness was heated at 140℃and equilibrated for 10 min.A constant axial force(2 N)was applied to the sample to maintain good contact between the sample and the parallel plates,and the relaxation modulus versus time was recorded under a constant strain of 5%.

2.2.2.2Self-healing property

The self-healing property of the samples were evaluated using an AI-7000-AGD testing instrument(Gotwell,China).The samples were cut perpendicular to the tensile direction and then connected to the fracture surfaces.Healing was performed for 5 min at 110℃ and 10 MPa,and subsequently tested.A stretching rate of 5 mm/min was used at room temperature.The self-healing efficiency was quantified by comparing the fracture stress ratio of the healed sample to that of the pristine sample,as the following equation.

2.2.2.3Thermal repair property

The thermal repair property of the samples were investigated by monitoring the recovery of the scratches using an Olympus BX51 microscope(Olympus,Japan).The film was scratched to form a crack on the surface using a razor blade,and the width of the crack(D1)was measured using a microscope.The film was then placed between two tin plates,and two clips were used to fix the tin plates in place.The samples were placed in a convection oven at 130℃for 5 min,and the width change of the crack(D2)was measured using a microscope.The thermal repair rate(Rr)was calculated using the following equation.

2.2.2.4Reprocessing property

The reprocessing properties of the samples were characterized by measuring the changes in the mechanical properties before and after processing.The samples were cut into tiny fragments and hot pressed in a mold to reshape.The mechanical properties of the reprocessed samples were tested using an AI-7000-AGD testing instrument.All samples containing DMAPwerereprocessed at 130℃and 10 MPafor 1 h.

2.2.2.5Shape memory property

The shape memory property of the samples were characterized using a heating and cooling process.

2.2.2.6Degradation property

In a 20 mL vial,the as-synthesized sample of 200 mg was added to 10 mg DMAPand 5 mL ethylene glycol.The mixture was then heated on a hot plate at 140℃and thedegradation property wasinvestigated.

3 Results and discussion

3.1 Characterization of the LNIPUs

In our previous study[18],we synthesized LNIPUs with tunable properties through a two-step method.First,the NPHU prepolymers were formed by reacting BCC and excess D-400 in a ring-opening reaction.Subsequently,LEHL were reacted with the unconsumed amino groups of the NPHU prepolymer through a Schiff base reaction.The obtained LNIPUs were subsequently molded into different shapes under thermal conditions for further characterization.The structural,mechanical,and thermal properties of the different LNIPUs were fully characterized.In this study,the effects of varying the LEHL content on the relaxation,self-healing,thermal repair,reprocessing,shape memory,and degradation properties of the LNIPUs were comprehensively investigated(Scheme 1).

Scheme 1 Schematic illustration of the LNIPU preparation process

3.2 Relaxation property of the LNIPUs

The effect of a change in the LEHL content on the relaxation property of the LNIPUs was investigated.Fig.1 shows the evolution of the relaxation modulus at 140℃;the characteristic relaxation time(τ*)was determined as the sample relaxed toG/G0=1/e of its initial modulus.All the samples exhibit clear stress relaxation behavior at 140℃.Sample BL50:50,containing 45.53%LEHL,shows aτ*value of only 9 s,which is lower than that reported in other studies[20−22].Furthermore,according to previous reports,because the complex macromolecular structure of lignin restricts chain segment motion of the crosslinked network,slower relaxation rates are always obtained when increasing the amount of lignin in LNIPUs[22].However,in this study,the relaxation time of the LNIPUs gradually decreased with an increase in the LEHL content.This is probably because an increase in the LEHL content promotes the transcarbamoylation and transesterification reactions in the network of the LNIPUs[23].These results are further confirmed in the following sections.

Fig.1 Stress-relaxation curves of the LNIPUs at 140℃.The dotted lines at G/G0=1/e represent the characteristic relaxation time(τ*)

3.3 Self-healing property of the LNIPUs

Owing to the dynamic covalent bonds in the crosslinked network,the LNIPUs can be healed under a thermal stimulus.To explore the self-healing property of the samples,the original samples were cut perpendicular to the tensile direction and the fracture surfaces connected;the healing process was conducted at 110℃and 10 MPa for 5 min.As shown in Fig.2,the healed sample forms with no discernible interface between the overlapped areas.Tensile testing shows that self-healing efficiencies of 75%and 68%were achieved for samples BL70:30and BL50:50,respectively.Furthermore,the self-healing efficiencies of the LNIPUs decrease with a further increase in the LEHL content.These results suggest that excess lignin hinders the movement of the polymer chain segments,restricting recovery of the covalent network[22,24].

Fig.2 Self-healing display of BL 70:30 and BL 50:50(a),stress-strain curvesof theoriginal and self-healed BL 70:30(b)and BL50:50(c)

3.4 Thermal repair property of the LNIPUs

To investigate the effect of varying the LEHL content on the thermal repair ability of the LNIPUs,the surfaces of the samples were scratched,and the widths of the scratches were observed under a polarizing microscope.Optical images of BL70:30and BL50:50were obtained during testing by making use of a microscope,as shown in Fig.3.After conducting a heat treatment at 130℃for 5 min,Rrvalues of 83%and 93%were achieved for the scratches on samples BL70:30and BL50:50,respectively.The results indicate that the LNIPUs has excellent thermal repair property,which is better than those of previously reported lignin-based vitrimers[20−21].This can be attributed to the highly efficient dynamic exchange reaction at the scratch surface of the LNIPUs under external thermal stimulation.

Fig.3 Thermal repair of BL 70:30(a1,b1)and BL 50:50(a2,b2)at 130℃for 5 min imaged using apolarizing microscope

3.5 Reprocessing property of the LNIPUs

The fragmented pieces of the LNIPUs can be reprocessed into shape by hot pressing in a mold.In our previous study[18],we tested the reprocessing ability of the LNIPUs using a DMAP catalyst.The materials were cut into pieces and treated at 130℃and 10 MPa for 1 h.To comprehensively investigate the effects of varying the LEHL content and cycle processing on the reprocessing property of the LNIPUs,samples BL70:30and BL50:50were subjected to a tworound hot-pressing treatment and further examined.As shown in Fig.4 and Table 1,the LNIPUs maintained a tensile strength higher than 75%after two hot-pressing treatments.Furthermore,a comparison between thetensile strengths of samples BL70:30and BL50:50indicates that the tensile strength of the LNIPUs clearly decrease with an increase in the LEHL content after two hotpressing treatments.In addition,the tensile strength slightly improves after the second hot-pressing treatment.These results indicate that the LNIPUs are mostly healed after two hot-pressing treatments.A reprocessing mechanism for the LNIPUs is proposed in Scheme 2.The carbamate and ester bonds can undergo transcarbamoylation and transesterification reactions at elevated temperatures in the presence of a DMAP catalyst,leading to rearrangement of the chemical crosslinking networks.This endows the LNIPUs with good reprocessing property.

Scheme 2 Proposed reprocessing mechanism for the LNIPUs

Fig.4 Stress-strain curvesof samples BL 70:30(a)and BL 50:50(b)after thefirst and second hot-pressing treatments

Table 1 Comparison of mechanical properties for BL70:30 and BL50:50 after two hot-pressing treatments

3.6 Shapememory capability of the LNIPUs

In addition to the self-healing and reprocessing abilities of the LNIPUs,they demonstrate a shape-memory capability.Shape memory polymers typically contain hard segments responsible for their permanent morphology, while the soft segments allow deformation above theirTg[25].Here,lignin acted as a hard segment for rapid and full shape recovery when the LNIPU film was molded in the rubbery state of lignin[26]. The thermoresponsive shape memory property of the LNIPUs were investigated by a simple heating and cooling process.As shown in Fig.5,sample BL90:10(flat shape)was first heated at 80℃for 10 min and then temporarily fixed in the"U"shape by a subsequent cooling process at room temperature.When the temperature was increased to 80℃again under stress-free conditions,the"U"shape rapidly reverted to its original flat state owing to the rubbery state of the NPHU prepolymer and lignin.Subsequently,after deformation and shape fixing at 80℃,the flat shape could further be reshaped into a"helix" shape,and the flat shape subsequently recovered by following the same"heating-cooling"procedure.

Fig.5 Imagesof the shape memory behavior of the LNIPUs(BL90:10)

3.7 Degradation property of the LNIPUs

Transcarbamoylation exchange reactions in PUs can often depolymerize the macromolecular chains into small molecules in the presence of small amounts of alcohol[27].As shown in Fig.6,sample BL50:50of 200 mg was mixed with 10 mg DMAP and 5 mL ethylene glycol,and the mixture was heated at 140℃for 20 h.The as-synthesized LNIPUs mostly dissolvedin ethylene glycol in the absence of any organic solvents.De-crosslinking occurs as a result of the alcohol participating in a dynamic reaction at elevated temperatures(Fig.7)[28−30].Future studies will be conducted to optimize the de-crosslinking conditions and to purify and characterize the alcoholysis products.

Fig.6 Alcoholysis-triggered degradation of BL 50:50 in ethyleneglycol

Fig.7 Mechanism of the alcoholysis-triggered degradation of BL 50:50 in ethyleneglycol

4 Conclusions

In this study,the polymeric properties of poly(iminehydroxyurethane)s(LNIPUs),tuned by varying the ratios of bis(6-membered cyclic carbonate)(BCC)and levulinate enzymatic hydrolysis lignin(LEHL),were comprehensively investigated.The LNIPUs exhibit a short relaxation time, good self-healing and reprocessing properties, and demonstrate thermoresponsive shape memory and alcohol-triggered degradation properties.A relaxation time of only 9 s and thermal repair rate of 93%was achieved for the LNIPU containing 45.53%LEHL.This study presents a novel strategy for the synthesis of thermally healable and re-processable LNIPUs based on dynamic chemistry,which could be good candidates for selfhealing and recyclablenon-isocyanatepolyurethanes.

Acknowledgments

The authors wish to express their gratitude for the grants from the National Key Research and Development Program of China(2017YFB0307903),Natural Science Foundation of China(21706154),and Foundation of Key Laboratory of Pulp and Paper Science and Technology of the Ministry of Education of China(KF201916).