Timothy G. Rials. and Wolfgang G. GlasserDepartment of Wood Science Bnd Forest Products and Polymer MateriBls and InterfacesLaborBtory. Virginia Polytechnic Institute and State University, Blacksburg,VA 24061. USA(Received 19 January 1988; revised 1 May 1989; accepted 3 July 1989)

The incremental elimination or hydroxy functionality in an organosolv lignin by ethylalion or acetylationdramatically innuenced the state of miscibility and resulting morphology or blends prepared withhydroxypropyl cellulose (HPC). A maximum level of interaction between the blend components, asdetermined from melting point depression, oa:.urred where 23-40-1. or the hydroxy groups were substituted.Above this level of modification, the interaction parameter decreased rapidly. Complete incompatibilityoccurred at a 90% degree of substitution with acetoxy functionality. Essentially three distinct phasemorphologies could be detected in these materials. At the lowest levels of interaction, the morphologyresulted from lignin domain formation and hydroxypropyl cellulose liquid crystal mesopbase ~paration.A completely miscible, amorphous blend resulted when the polymer-polymer interaction was maximized.At intermediate levels, however, these materials were characterized by a dispersion of liquid crystal domainsin an amorphous, lignin-reinforced HPC matrix.

(Xey-ora: bydrox)"prop)i ffilalose; &cilia; &quid ~r,s(8Is; poIyater bkMs)

use in such materia) systems has received little attentionin the open literature. Ternary phase behaviour studies ofcellulose/cellulose acetate 7 and ~lIulose acetatel'hydroxy-

propyl cellulose' in solution have been reported, but werenot extended to the bulk morphology of the blend. Conse-quently, the phase behaviour of these polymer systemsrelative to blend morphology rrcmains an important,unanswered question.

Studies on the morphology and properties of hydroxy-propyl cellulose (HPC) blends with small amounts oflignin have shown both modulus and tensile strengthincreases in excess of 1500/.9. For this partially misciblepair, it was shown that a fibrous HPC phase is dispersedin an amorphous matrix of lignin reinforced HPC.Considering the highly polar character of both hydroxy-propyl cellulose and lignin, it is plausible that secondaryinteractions between the component polymers contributeto the overall state of miscibility in this binary system.The intent of this work is to address further the innuenceof intermolecular interactions on the phase morphology ofH PCflignin blends by modifying the hydroxy functionalityof the lignin.

INTRODUCTION

Some of the more significant recent developments inmaterials technology have come in the area of multi-component polymer systems. These developments havebeen led by the success of high strength, light weight fibre-reinforced polymers, although considerable attentionremains focused on polymer blends, which offer thepotential of much simpler fabrication technology'. Onenovel ap:proach, hoping to combine the desirable attri-butes of these two material classes. has been the use ofa rigid rod polymer as one component of the polymerblcnd. thereby replacing the macroscopic fibre with asingle polymer chain2. Conceptually, this would retainthe inherent high strength of fibre composites andeliminate such detrimental features as a weak intcrfaceand property anisotropy.

Consistent with Flory's prediction of phase separationin a mixture of a rigid rod and a flexible polymerJ, thetrue molecular composite has yet to be produced. Thefeasibility of this type of blend system has been clearlydemonstrated, however, with liquid crystal (LC) copoly-esters serving as rigid rod polymers in various matrices.'s.Appropriate processing allowed orientation of the LCphase, resulting in dramatic strength increases in thesebinary systems. Surprisingly, although cellulose and itsderivatives also exhibit liquid crystal phenomena6, their

EXPERIMENTAL

MaterialsHPC (Klucxl 'L ') used in this study was supplied by

Aqualon Inc., Wilmington, DE, USA. The manufacturerreported a molar substitution of four propylene oxideunits per anhydroglucose unit and a nominal molecularweight of IO5gmol-l.

. Current addraa and to whom conespondencc should be addressed:

USDA Forest Service. Southem Forest Experimental Stltioft. 2,SOOShreveport Highway, PinevilJe, LA 71360. USA

POLYMER, 1990, Vol 31, July 13330032-3861~7133~C> 1990 Butterworth.HeiMmann Ltd.

The lignin component was an organosol v ligninisolated from aspen wood, and supplied by BiologicalEnergy Corporation of Valley Forge, PA, USA. Thepolystyrene equivalent number «M.»and weight «M.»average molecular weights were determined by gelpermeation chromatography as 900 and 3000gmol-I,respectively. The hydroxy content of the lignin wasincrementaJly eliminated by acetylation and ethylationaccording 10 previously described procedures I °. Tobie I

presents a summary of the hydroxy data, and pertinentphysical properties, of the derivatiud lignins.

.~.. .

;::

e~R~~ ./.w'

~

-,-

~~

~ --

.Y

V J10 40 10 ~ 1]0 .0 1.0

TEMP£~AT~ I"CI

FiIun I D.s.c. CUrvel for HPC blendl with ullmooirlCd orpnosolvlipin. The -'ei,ht fr8CtiOll (%) of lipiD in the bIcad is Iho.'n on thecu~

Mt'thodsIndividual solutions of the blend components in

dioxane (or tetrahydrofuran) were mixed and stirred for~ 12 h before casting into a Teflon mould, Solventevaporation proceeded under ambient conditions for 24 hfollowed by transfer to a vacuum oven at 6O"C for furtherremoval of solvent. The dried films were then stored ina vacuum desiccator over P2O,.

The glass transition (T.) and melting (Tal) tempera-tures of the component polymers and their blends weredetermined on a Perkin-Elmer DSC-4 interfaced to thethermal analysis data station. All materials were analysedat a heating and cooling rate of lOoC min - 1 under a

purge of dry nitrogen. Dynamic mechanical properties(1og E' and tan~) were determined with a PolymerLaboratories Inc. dynamic mechanical anai)'ser inter-faced to a Hewlett-Packard microcomputer. The spectrawere collected at a heating rate of 4°C min -I from - SO to

ISOoC using a single cantilever beam geometry.

above that or the pure lignin component. and somecomposition dependence remains in the lower T.. Theabove results suggest possible phase separation or thehigh molecular weight lignin species. and is consistentwith the overall picture or partial miscibility ror thispolymer pair.

The evaluation or the amorphous ph.ase behaviour rorthis blend system is complicated by the presence or anadditional second-order transition (T l) present in HPC.Occurring at sooC, this transition has been attributed to

a residual liquid crystal phase in the bulk polymer..) In an effort to assess more accurately the behaviour or

these two phases upon blending with lignin, dynamicmechanical analysis was used to characterize the relax-ation properties. As seen in Figll'~ 2, the tan d spectrumor HPC consists or two primary relaxations, one at 30and one at 85°C, which correspond to the T. and T ltransitions observed by calorimetry. As the lignin compo-sition or the blend increases, the intensity or the T l

RESULTS

Unmod!/i~d lignin (OSL) blends

Figur~ 1 presents a series of therrnograms for blendsprepared from HPC and the unmodified organosolvlignin. As the lignin content is increased to 4Owt%, asingle T. is observed, increasing from about 25 to rooC.The increase in 7: is accompanied by a substantiaJdecrease in the me'ting temperature of approximately4OcC. As the lignin content is increased to SS8/e, anincrease in T. is observed and two glass transitions arepresent at 65 and 1)o°C, indicative of phase separationof the two components. Interestingly, the higher tempera-ture transition in the phase separated systems is slightly

1334 POLYMER. 1990. Vol 31. July

lorLO9 ['I :.: -. -:.~:~

"~""

0.4040~,'" \55~

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i" \.1. \ton ~ , , \ ;

I l',. I{I \'

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l! i-, ,,1 '1 "I ,,... " .

. /0% ",,/!' i'/ -- I "v,-- I /-

-- -,-,,_:=.::~-::::.~.,.'"--

-50 0 50 100 150

TEMPERATURE COCI

ficure 2 Dynamjc mechanical properties of HPC blends preparedwith unmodified organosolv li&nin. The numbers represent the weightrraction or lignin in the blend

0.14

0.081

Multiphase materials with lignin. 5: T. G. Rials and W. G. Glasser

sharper in the initial scan of this blend system. Althoughthis is speculation, at best, this may arise as a consequenceof molecular weight fractionation ofHPC in these blends.

After this initial thermal treatment, d.s.c. analysis(Figure Jb) reveals an. enormous increase in the heatcapacity change (AC p) at T" implying an increase in theamorphous volume fraction of the blends. While ofincreased intensity. the transition is seen to occur overa narrower temperature range as the lignin contentincreases. It is impossible to attribute the dramaticenhancement of the amorphous component to the partialextinction of a crystalline phase comprising only 16% ofthe pure HPC. In accordance with previous results', asecond HPC phase of intermediate order (i.e. related toa liquid crystal mesophase) must be invoked, and is bestevaluated through the blend's dynamic mechanicalproperties. Figure -4 illustrates the relaxation behaviour ofthe HPC/Et-3 blend system, which exhibited behavioursimilar to the highest ethylated lignin blends, althoughnot to the same extreme. As the lignin content is increasedto 20-10, the T 2 relaxation is shifted from 100 to sooC. Therelaxation also increases in intensity while decreasing inbreadth, suggesting a more homogeneous environment.

The conclusion to be drawn from this experiment isthat the modification of lignin by ethylation yields blendsranging from partially miscible to miscible. As the degreeof modification increases, the compatibility with HPCincreases, leading to a complete disruption of super-molecular structure and an essentially amorphous blendafter thermal treatment.

Acetylated lignin (Ac) blendstransition apparently increases at the expense of the ~ acetylat.ed lignins p~ov~de a much more eompr:-amorphous phase relaxation. However, closer inspection henslve range, In that substitutIon levels from 23 to 100 '10

reveals that this behaviour actually arises from a tempera-ture increase in the T" which results in a furtherconvolution of the two observed relaxation processes.The conclusion that can be drawn from these results isthat partial compatibility exists between lignin (un-modified) and the amorphous phase of HPC without asignificant effect on the formation, and separation, of aliquid crystal mesophase.

1,I RIo

~..~

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/y

~

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~~

~,Eth.\'1ated lignin (Et) blends

The purpose of ethylation is conversion of lignin'sphenolic hydroxyl groups with very little impact on thealiphatic functionality of the molecule. For HPC blendsprepared from the ethylated lignins a singJe. unique T.was found over the entire composition range investigated.At low levels of ethylation (Et-1 and Et-2. Table J) thethermal behaviour of the blends ,,'as essentially identicalto that of the unmodified lignin materials. In the blendsprcpared from the more highly ethylated lignin derivatives.however. some unusual properties were encountered. Themost extreme thermal behaviour. as revealed by differ-ential scanning calorimetry (d.s.c.), is found in the blendsprepared from the highest substituled lignin. Et-4 (Figure30). Even the lowest lignin composition blend providesa striking conlrast to pure HPC. Perhaps the mostnotable distinction between HPC and these blends is theapparent resolution of the K - Nand N - I transitions.and the dramatic T. depression of almost WC at only5% lignin content. As the lignin content is increased. themelting point depression continues with the maintenanceof about a J 5°C window between the two first-ordertransitions. In addition. the T 2 transition appears much

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/ . . . . . . . !10 40 70 100 130 160 190 220

TEMPERATURE I-CI

nc- 3 D.$.~. analysis or HPC/Et-4 blends; (a) initial _n; (b)immediate re.scan. The numbers represent the weiaht rfaction orelbylated lignin in lhe blend

POLYMER. 1990. Vol 31. July 1335

Multiphase materials with lignin. 5: T. G. Rials and W. G. GI.sser

initial increase followed by a substantial rcduction incompatibility between blend components as the degreeof acetylation increases. An evaluation of the T, of thesematerials revealed a single transition over the blendcompositions investigated until at the two highest levelsof substitution the T,s of the individual polymers weredetected, indicating classical phase separation.

It is of interest to return briefly to the blends preparedwith the low OS lignin (23 wt8J.). The analysis of thisblend series by d.s.c. (Figur~ 6) reveals behaviour similarto the Et-3 and Et-4 blends. That is, even at relativelylow lignin compositions, a tremendous disruption occursin the development of LC superstructure. As the lignincontent increases from 10 to 20% of the blend, aD tracesof crystallinity disappear and the T,s become muchsharper, and more classical, in their appearance. Theimplication is that, at this level of substitution oflignin hydroxy functionality, the increased compatibilitybetween the two polymers leads to an essentially amor-phous material with no evid~ of LC mesophaseformation.

Melting point depressionFrom the data presented thus far. it is evident that

lignin modification yields polymeric blends varyingwidely in their morphology and properties. By inference.these differences are attributed to changes in componentmiscibility; however, they provide only a qualitativeindication of the relationship existing between ligninstructure and blend compatibility. Further insight intothis question can be gained through closer inspection ofthe T. depression in the blend. For semi-crystallineblends such as these. the polymer-polymer interactionparameter, 8, can be determined through the followingsimplified expressionl':

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3

\\.

\ ~:::~"oQ'~~~ -0 ~ ~\ ~ I::. ,,~

b 65~

'. 23~* ~ .a ~ 25 to ;5 Io~

LIGNIN C~TENT (WI. '1.1

FI&- 5 The variation ill ~t. tryJtallinilY wilh IiJDin contentfor HPC blends prepared with a"tylatcd lianin derivatives. Thenumbers IepleStnt the degree 01 substilution of h)'droxyl groups

were achieved (Table 1). It is, therefore, not surprisingthat a much wider range of blend morphologies wereobtained. Figure j presents the variation in crystallinity(determined by d.s.c.) of the blend as the degree ofacetylation increases. At low levels of modifICation(230/.), the crystallinity in the blends is rapidly diminished,becoming undetectable at a composition of 10./. lignin.Further substitution, however, results in a greater levelof crystallinity being retained in the blend until at degreeof substitution DS = 870/. the percentage crystallinityremains hisher than that or blends made with theunmodified parent lignin. Sin~ the d~se in crystallinityof the blend may be attributable to the interaction ofblend components, this information translates into an

1336 POLYMER, 1990, Vol 31, July

I

lu

where T~2 is the equilibrium melting point of H PC( = 213.1 'C. from Reference 16). T.2 is the melting pointof the blend. ~H 2.iV 2. is the heat of melting per unjtvolume of 1000/. crystalline material (= 7.52 cal cm - 3.

from Reference 16) and 4>, is the volume fraction of theamorphous blend component (i.e. lignin). A plot of ~ T.2

Multiphase materi81s with lignin. 5: T. G. RiBls and W. G. Glasser

Sample

HPCI'OSL

HPC',!Ac.2HPC1Ac-)HPC:Ac-4HPC,'Ac-S

HPC/'EI-IHPC,'E1-2HPC/'Et-)---

1-.. 5IGpeI'

0.980.950.980.910.94

0.990.970.91

-B

(calcm-JI

9.67S

2.3223.187S.3376.10.

S.2I1S.SI4S.212

196.1

41S.628S.S102.497.1

3SI.916S.6420.7

3.0

6.454.431.591.52

5.462.~6.52

both blend components make it impossible to accuratelydefine the parameter B. Yet the assessment or meltingpoint depression using this approach does allow therelationships existing between lignin structure and blendmorphology to be clarified. As seen in FiglU~ 7, areduction in the hydroxy content or lhe lignin componentincreases lhe level or interaction occurring bel ween thetwo polymers until a maximum is reached at a pointcorresponding to DS~408j. (represcnlcd by the Ac-1 andEl-4 blends, which were largely amorphous, making itimpossible to determine the TID depression). Furtherreduction of the lignin hydroxy runctionality results in adiminished interaction until at complete substitution anincompatible system is found (Ac-S). It should beemphasized that this curve is generated from data ODboth the acetylated and ethylated lignin blends; conse-quently, the overlap that is encountered in this compositecurvcsuggests that the type ofmodiflC8tion used does DOtsubstantially influence the polymer-polymer interaction.Furthermore, this trend indicates that the hydroxyfunctionality of lignin does not favourably contributeto the miscibility of the two components by estab-lishing intermolecular hydrogen bonds. This result isemphasized upon considering that the maximum inter-action occurs at a degree of substitution which providesa very close match in component solubility parameters(10.0<611 Etl0.S; 6HPC= 10.1).

A closer evaluation of the system as a whole pro\idcssome interesting insight into the factors contributing tothe origin of supermolecular structure in this polymericblend and, subsequently, impacting the technologyrelated to the development of high strength compositematerials. At low levels of interaction between the lignincomponent and HPC (B> -2.0), phase separationoccurs due to the unfavourable energy bet,,'een thepolymers, as well as liquid crystal mcsophase formationby HPC. This necessarily has a catastrophic effect onmaterial properties due to the heterogeneity of thematerial. At the other extreme (B<-7.0), a miscibleblend is obtained, which presumably represents the idcalsituation; however, it appears as though the rigidit~. ofthe cellulosic chain is derived largely frl)m intermolecularchain interactions. The disruption or these interacti"nsthat results upon comp3tibilization ,,'ith a second polymcryields a much more nexible polymer. eliminalin~ theopportunity for high strength development. In the inter-mediate range of polymer interaction (- 7.0 < B < - ~.O),the blend morphology originates solely rrom LC meso-phase formation dispersed in a miscible HPC!1igninmatrix. It is with this type of phase morpholo~y thatsignificant enhanccmcnts in both modulus and tensilestrength of the HPC;lignin composite have been noted°.

rersus ~: should then yield a straight line with a slopeproportional 10 B. The results of this treatment for theHPC,lignin blends are presented in Table 2.

For the HPC blends incorporating the unmodifiedlignin. B= -3.0. indicating a favourable interactionbet".een the t".o polymers. Upon acetylation. the magni-tude of B initially increases before falling off to- 1.52 cal cm -) upon complete substitution. ".hich is

consistent ~.ith the observed morphologjes presentedearlier. With the exception of the Et-2 blends. the degreeof interaction in blends of the elhylated lignins moreclosely resembles the lo~' DS. acelylaled materials. Thatis. an extremely favourable interaction is indicated. inaccord ~'ith earlier conclusions regarding the misciblenature of these blend systems.

Although all of the blend systems are adequatelydescribed by this linear model (0.91~R2<0.99). il isinteresting that the intercept fails 10 pass through zero.as expected. for all cases. Generally. Ibis behaviour hasbeen attributed 10 aT. depression arising from entropiccontributions such as a reduction in lamellar thicknessof the crystallites 15. Since no attempt was made here toobtain equilibrium melting points. this certainly may bethe case; however. the discrepancy is greatest for blendsystems identified either as partially compatible (e.g.unmodified lignin) or incompatible (e.g. Ac-4/5). Thedeviation from zero may thus be indicative of incipientphase separation of the pol)'Jner pair.

CONCLUSIONS

The elimination of lignin's functionality significantlyinfluences the state of miscibility that exists ~'hen il isblended with H PC. A maximum interaction bet~'een thetwo polymers occurs al a degree of substitution bet~.een30 and 40%, irrespective of the type of modification (e.g.ethylation or acetylation). Continued reduction of hydroxyfunctionality diminishes the H PC/lignin interaction until acompletely incompatible system is obtained at OS =908/8.Partial miscibility of this binary system yields a com-patible, amorphous phase interspersed with H PC liquidcrystal domains which, upon appropriate processing,

DISCUSSION

Uncertainties associated with the characterization of

POLYMER, 1990, Vol 31, July 1337

Multiphase materials with lignin. 5: T. G. Rills Ind W. G. Glasser

develops high strength materials. Surprisingly, as com- 3 f1ory, P. J. J. C.".. P~,.,. 1949. 17, :MI3patibility is maximized, the rigidity of tbe ttliuJose chain 4 Siclman, A~ Dapn. A. and KtniJ, S. PoI}WIfr 1915, _,1326is losl, as is the mechanical inlegrity of the polymer 5 Pad, M., Barouc. C. and M81agnini, P. J. Pol"",. Sri., Pol, .

Th I d. I . bed I p~)'S.U..19I7,15.1595composIte. esc resu ts lrect y Impact t eve oplnent 6 Gray, D. G. in 'Polymeric: Liquid Crystals' (Ed. A. Blumstein).of cellulose-based polymer composItes. Plenum Plea. New York, 1915. p. 369

Maruno, E.. Bianchi. E., Citrrri. A., Ramis. G. and TaJdi. A..\#«rt-'«v1ls 1916. .'.626Marsano, E~ Bianchi. E. and Cikrri. A. M«r~ 1916.17.2886Rials. T.G.andGlaWE. W.G.J.A",.P~,...Sci.I989,37.2399Rials. T. G. P~.D. Tfwsis, Virlinia Polyt«hnic Institute andState UniYenity, Blacksburg. VA, 1916Small. P. A. J. A"... Ca. 1953,3,71Aspler. J. S. aDd Gray, D. G. PoI,-. 1912, 23.43Navanl, P. aDd Hardis, J.-M. in 'Polymeric Liquid CrysIaII'(Ed. A. Blumstein), Plenum Press, New York, 1915, p. 389Kya. T ~ MukMija, P. aDd Park, H.-S. M«r~c:vIu 1915.18,2331Harris. J. E~ Paul, D. R. and Barlow, J. W. iJI'POI)'DIer BlelldaaDd Com~ iD Multi~ S",cmI' (Ed. C. D. Hae),Amrricu ChaDaI Socicty, WashillatOD DC, 1914, p. t7Samuek,I.. J. J. Pol,.. Sd. 1969, .4.-1,1197 .

.,

,10II1213

1415

ACKNOWLEDGEMENTSThe authors gratefully acknowledge the financial supportprovided by the National Scien~ Foundation undergrant no. CBT-8S 12636.

REFERENCES

I KlaDtz, F. G. i8 'Multi(X)mpoucal p~ 5711-' (Ed. I.. F.Gould), AmericaJI a.anaJ scaly, WulliJlltoa DC, 1911,p.4S2

2 Hwan.. W. F.. WiI, D. R.. VerM:hOOfe, C., PM, G. E..HeImiajat, T. E. aDd Ad.-. w. W. Pol,.. E.,. sa. 1M3, 23,714 . ,-:.C~ 16

1338 POLYMER, 1990, Vol 31, July