Polymer. Effects of molecular weight on poly(u-pentadecalactone) mechanical and thermal properties

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1 Polymer 1 (21) Contents lists ville t ScienceDirect Polymer journl homepge: Effects of moleculr weight on poly(u-pentdeclctone) mechnicl nd therml properties Jili Ci, Chen Liu, Minmin Ci, Jie Zhu, Feng Zuo, Benjmin S. Hsio, **, Richrd A. Gross, * NSF I/UCRC for Bioctlysis nd Bioprocessing of Mcromolecules, The Polytechnic Institute of New York University, Six Metrotech Center, Brooklyn, NY 1121, USA Deprtment of Chemistry, Stony Brook University, Stony Brook, NY 11794, USA rticle info strct Article history: Received 17 Novemer 29 Received in revised form 4 Jnury 21 Accepted 7 Jnury 21 Aville online 14 Jnury 21 Keywords: Poly(u-pentdeclctone) Moleculr weight Tensile Aseriesofpoly(u-pentdeclctone) (PPDL) smples, synthesized y lipse ctlysis, were prepred y systemtic vrition of rection time nd wter content. These smples possessed weight-verge moleculr weights ( ), determined y multi-ngle lser light scttering (MALLS), from to Cold-drwing tensile tests t room temperture of PPDL smples with etween nd showed rittle-to-ductile trnsition. For PPDL with of , inter-firillr slippge domintes during deformtion until frcture. Incresing ove resulted in enhnced entnglement network strength nd strin-hrdening. The high smples lso exhiited tough properties with elongtion t rek out 6% nd tensile strength out 6.8 MP, comprle to liner high density polyethylene (HDPE). Reltionships mong moleculr weight, Young s modulus, stress, strin t yield, melting nd crystlliztion enthlpy (y differentil scnning clorimetry, DSC) nd crystllinity (from wide-ngle X-ry diffrction, WAXD) were correlted for PPDL smples. Similrities nd differences of liner HDPE nd PPDL moleculr weight dependence on their mechnicl nd therml properties were lso compred. Ó 21 Elsevier Ltd. All rights reserved. 1. Introduction Polyethylene is the most widely used commodity polymer. It is found in mny consumer products, such s milk jugs, detergent ottles, mrgrine tus, grge continers, wter pipes, just to nme few. Poly(u-pentdeclctone) (PPDL) is new type of thermoplstic tht cn e synthesized y lipse ctlysis [1 3]. The chemicl structure of PPDL, with 14 methylene groups nd n inchin ester linkge in ech repeting unit, is very similr to tht of liner high density polyethylene (HDPE) (Scheme 1). Polyethylene (PE) cnnot e esily decomposed into smll molecules fter usge. To chieve extensive degrdtion of the PE cron ckone, tretment of PE with strong oxidized gents such s nitric cid [4], ozone [] nd permngnic cid [6] or pyrolysis t high rection tempertures (ove 37 C) [7,8] is required. Therefore, white pollution [9,1] from un-recycled PE plstics is mounting prolem tht mnkind must confront. An importnt dvntge of PPDL over PE is tht the former hs ester groups in the ckone tht re susceptive to chin rekge. Consequently, gentle * Corresponding uthor. Tel.: þ ; fx: þ ** Corresponding uthor. E-mil ddress: jlci24@yhoo.com (R.A. Gross). enzymtic hydrolysis cn, in principle, e used to decompose PPDL ck into monomer uilding locks. Currently, on-going studies re in progress to find suitle enzyme for PPDL iologicl recycling oth in our lortory nd elsewhere [11]. Chemicl ctlysts such s potssium lkoxides [12], diethylzinc [13,14] nd yttrium isopropoxide [1] cn e used for conversion of mcrolctones to polyesters. For u-pentdeclctone (PDL), the use of immoilized lipse ctlysis hs een proven to e superior to chemicl ctlyzed routes for mking polymers, resulting in more rpid polymeriztion kinetics s well s yielding polyesters of reltively higher moleculr weight. Immoilized lipse-ctlyzed polymeriztion of mcrolctones ws first pulished y Uym et l. [16]. Our lortory demonstrted tht, using Novozym 43 tht consists of Cndid ntrctic lipse B (CALB) physiclly immoilized on mcroporous support, the polymeriztion process of PDL gives PPDL with numer-verge moleculr weight (M n )upto in yields exceeding 9% (route is illustrted in Scheme 1) [17]. Thus, enzyme-ctlyzed preprtion of PPDL with high moleculr weight hs provided suitle mterils for evlution of their physicomechnicl properties. For exmple, mechnicl properties of PPDL with M n (polydispersity, /M n,2.)[18] nd thiol-functionlized PPDL telechelics [19] hve een studied. Previous crystllogrphic work [2] indicted tht the nd prmeters of PPDL unit cells re slightly lrger thn those of PE s /$ see front mtter Ó 21 Elsevier Ltd. All rights reserved. doi:1.116/j.polymer

2 J. Ci et l. / Polymer 1 (21) CH 2 CH 2,Chromium/silic ctlysts, Ziegler-ntt ctlysts c,metllocene ctlysts High density polyethylene CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -(-CH 2 -CH 2 -) n --- Poly(ω-pentdeclctone) ( CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -CH 2 -C-O-CH ) n 2 -CH O Toluene 7 C Enzymtic Ctlysis Synthesis Route O O CH 2 11 PDL Scheme 1. Comprison of moleculr structures of high density polyethylene (HDPE) nd poly(u-pentdeclctone) otined vi enzyme-ctlyzed ring-opening polymeriztions. nd the until cell prmeter long the fier xis is much lrger thn tht of PE s. DMA tests showed tht PPDL with M n hd glss trnsition t 27 C. Also, the oserved high storge modulus ws ttriuted to high crystllinity s determined y DSC nd WAXS [18]. PPDL-sed copolymers with other monomers including trimethylene cronte [2], p-dioxnone [3] nd u-cprolctone [21] were synthesized nd studied y therml nd X-ry nlysis. Copolymers were found to e highly crystlline rndom copolymers over the entire composition rnge. This ehvior ws ttriuted to co-crystlliztion of comonomer units in common lttice or isomorphic sustitution of comonomer units. The current study imed to investigte the effect of PPDL moleculr weight on its mechnicl, therml nd rheologicl properties. The chosen synthetic methods enled the preprtion of PPDL with vlues up to nd PDI vlues close to 2.. Films were prepred y press-molding t 13 C nd tensile testing ws performed on these smples. Bsed on the shpe of stress strin curves, rittle-to-ductile trnsition long with mximum elongtion t rek ws oserved. Since the chemicl structure of PPDL is similr to polyethylene (PE), its therml nd mechnicl properties were compred with those of commercilly otined PE smple [22]. In ddition, PPDL films with different moleculr weights were nlyzed y differentil scnning clorimetry (DSC), wide-ngle X-ry diffrction (WAXD), smll-ngle X-ry scttering (SAXS) nd dynmic mechnicl nlysis (DMA) to revel the moleculr weight dependence of mechnicl, therml, crystlline nd rheologicl properties. It ws found tht Young s modulus nd stress t rek exhiit distinct crystllinity dependence; in ddition, differences, of elongtion t rek nd true stress t rek, etween PPDL nd liner PE, re discussed. 2. Experimentl 2.1. Mterils Smples of u-pentdeclctone (PDL, 98%) nd nhydrous p-xylene (>99%) were purchsed from Aldrich Chemicl Co. nd were used s received. Chloroform ws purchsed from PHARMCO-AAPER Inc. (>99.9%). Anhydrous toluene (98%), purchsed from Aldrich Chemicl Co., ws dried over sodium nd then ws distilled under nitrogen. Novozym 43 (specific ctivity 1, PLU/g) ws gift from Novozymes (Bgsverd, Denmrk) nd consists of Cndid ntrctic Lipse B (CALB) physiclly dsored within the mcroporous resin Lewtit VPOC 16 (poly[methyl methcrylteco-utyl methcrylte], supplied y Byer) Smple preprtion Enzyme-ctlyzed synthesis of PPDL smples 1 3 (Tle 1) Pentdeclctone (PDL, 4 g) ws polymerized to prepre PPDL smples 1 3 t 7 C, with mgnetic stirring, in toluene (monomer:toluene ¼ 1:2 wt/v), using Novozym 43 s the ctlyst for predetermined rection times (see Tle 1). Rections were crried out in ml round-ottom flsks fitted with mgnetic stirrer nd n inlet/outlet for nitrogen gs. No precutions were tken to dry the monomer, ctlyst, solvent or glsswre. The rtio of

3 19 J. Ci et l. / Polymer 1 (21) Tle 1 PPDL nd PDI vlues for PPDL smples nd corresponding rection times for smple synthesis. Smple numer 1 4 PDI Rection time (h) c From light scttering. PDI is polydispersity index tht ws determined y GPC reltive to polystyrene. c Clcultion performed without inclusion of low moleculr weight peks oserved in the GPC chromtogrm (see Fig. 7). Novozym 43 to PDL ws 1% (w/w). All liquid trnsfers were performed y syringe through ruer septum cps under nitrogen. The finl product mixture ws dissolved in cold chloroform, filtered to remove ctlyst eds, precipitted in methnol to remove unrected PDL nd then solvent ws removed t C under vcuum for 16 h Enzyme-ctlyzed synthesis of PPDL smples 4 6 (Tle 1) The method used ws vrition of tht descried y de Geus [23] to otin PPDL of higher moleculr weight. PDL ( g) ws polymerized to prepre PPDL smples 4 6 in toluene (monomer:- toluene ¼ 1:2 w/v), using Novozym 43 s the ctlyst for predetermined rection times (see Tle 1). Rections were conducted in ml three-neck round-ottom flsks fitted with glss overhed stirrer nd connected to nitrogen nd vcuum lines vi fire stone vlve. The rtio of Novozym 43 to PDL ws 1% (w/w). Vcuum pressure in the system ws controlled (.2 mmhg) y J-KEM vcuum regultor. Novozym 43 ws dried for 16 h t C under 2 mmhg. The rection vessel contining 3 Å sieves ws predried t 1 C (mient pressure) for 16 h nd, therefter, ws flme dried nd the tmosphere ws switched to dry nitrogen prior to monomer nd solvent ddition. Glss syringes nd needles were pre-dried t 1 C for 16 h nd were cooled under nitrogen purge till they ws sufficiently cool to e hndled for solvent nd monomer trnsfers. Dry toluene (8 g) PDL (4 g) ws dded to the flsk y syringe under nitrogen. Then, one port on the vessel ws opened while purging with nitrogen nd dried Novozym 43 ws dded. Rection contents were heted y immersing the flsk in n externl oil th t 8 C. After the llotted rection time, 1 ml of pre-heted p-xylene (1 C) ws dded to the highly viscous slurry of enzyme, PPDL nd toluene. The slurry ws stirred for 1 h to dissolve PPDL nd then filtered using pre-heted Buchner-funnel. Smll liquots of pre-heted (1 C) p-xylene solvent were used to quntittively trnsfer rection products. The filtrte ws dded slowly to cold methnol (2 ml) to precipitte PPDL tht ws collected using Buchner-funnel. Finlly, the polymer ws dried under vcuum (2 mmhg) for 16 h t C to remove solvents Nucler mgnetic resonnce chrcteriztion Both 1 H nd 13 C NMR spectr were recorded t room temperture on DPX3 spectrometer (Bruker Instruments, Inc.) t 3 MHz in chloroform-d. Chemicl shifts (in prts per million) for 1 H nd 13 C NMR spectr were referenced reltive to tetrmethylsilne s n internl reference t.. All synthesized PPDL mterils descried herein hd identicl NMR spectr with signls nd ssignments s follows: [C(]O) CH 2 CH 2 c (CH 2 d CH 2 d ) CH 2 c CH 2 O] 1 H NMR (CDCl 3, d): 4.1 (t, J 6. Hz, CH 2 O); 3.8 (t, J 6. Hz, CH 2 OH); 2.24 (t, J 7. Hz, CH 2 CO); 1.9, 1.22 (rs, CH 2 c,d ) ppm. 13 C NMR (CDCl 3, d): (COCH 2 ), 64.4(CH 2 O), 34.4(OCOCH 2 ), , 28.6, 2.9, 2. (ll other crons) ppm Moleculr weight determintion To determine the solute moleculr weight of PPDL smples, Wytt HELLOS multi-ngle light scttering detector nd Wytt Optil rex differentil refrctive index detector were used. These two detectors were connected y steel tue with inner dimeter. mm nd CRZEL syringe pump. The solution concentrtion rnged from 1 4 to 1 3 g/ml. Astr V softwre ws used to cquire nd process the dt ccording to Zimm plots. The reltive moleculr weight of PPDL smples were determined y gel permetion chromtogrphy (GPC) using Wters HPLC system equipped with model 1 pump, model 717 utosmpler, nd model 41 refrctive index detector with, 1 3,1 4, nd 1 Å Ultrstyrgel columns in series. Wters Empower GPC softwre (Version 3, Viscotek Corp.) ws used for dt nlysis. Chloroform ws used s eluent t flow rte of 1. ml/min. Smple concentrtions nd injection volumes were.2% w/v nd 1 ml, respectively. The numer-verge moleculr weight (M n ) nd weight-verge moleculr weight ( ) were determined sed on clirtion curve generted y nrrow moleculr weight polystyrene stndrds (Aldrich Chemicl Co). GPC nd light scttering determined vlues of were similr (within 12%). For PPDL moleculr weight vlues reported herein, light scttering nd GPC /M n vlues were used (see Tle 1). 2.. Frctiontion of PPDL smple To seprte low moleculr weight mteril in smple, corresponding to GPC peks oserved t longer retention times, the smple ws frctionted s follows. A solution contining 2 ml of chloroform nd.2 g of Smple ws prepred. The mixture ws mintined t 2 C for 24 h with continul mixing y mgnetic stirring. The insolule PPDL frction ws seprted y filtrtion nd solvent ws removed in vcuum oven t 4 C for 7 dys, giving.17 g of insolule solid smple. Solvent removl from the solule PPDL frction ws crried out y rotor-evportion nd susequent drying in vcuum oven, where the recovered solule PPDL smple ws.3 g Tensile testing Dumell shped smple rs with dimensions of 2. mm (length) 4. mm (neck width) 1. mm (thickness) were prepred y press-molding t 13 C nd susequent quenching t mient temperture. An Instron 42 tensile testing mchine with N lod cell ws used for mechnicl study (the crosshed speed ws 3 mm/min nd the test temperture ws 2 C). The Merlin softwre ws used to collect nd nlyze the tensile results (stress ws clculted ccording to the initil cross-section re). The vlues of tensile strength, Young s modulus, elongtion t yield nd rek, stress t yield were otined y verging the dt otined from more thn 4 specimens Dynmic mechnicl nlysis (DMA) PPDL smples were molded into rectngulr rs with dimensions of 3 mm (length) mm (width) 1. mm (thickness). DMA mesurements were performed in single-cntilever ending mode using dynmic mechnicl therml nlyzer (DMTA) (Tritec 2 DMA, Triton Technology Compny). Mesurements were performed from 3 Cto9 C t heting rte of 2 C/min nd frequency of 1 Hz. Two identicl specimens with the sme moleculr weight were evluted nd results reported were men vlues. The Triton Technology DMA Softwre ws used to cquire nd process the dt.

4 J. Ci et l. / Polymer 1 (21) Differentil scnning clorimetry DSC mesurements were performed using differentil scnning clorimeter (Model 292, TA Instruments). Temperture clirtion ws crried out using n indium stndrd. Mesurements were performed under nitrogen tmosphere t flow rte of ml/min. Typicl prmeters for experimentl mesurements re s follows: i) smple cooled to 1 C, ii) heted to 1 Ct1 C/ min, iii) held t 1 C for 3 min nd then iv) cooled to 1 Ct1 C/ min. When temperture reched 1 C, the smple ws heted gin to 2 Ct1 C/min. The melting temperture, melting enthlpy, crystlliztion pek temperture, nd crystlliztion enthlpy were nlyzed y using the TA Universl Anlysis softwre X-ry mesurements Wide-ngle X-ry diffrction (WAXD) nd smll-ngle X-ry scttering (SAXS) experiments using the dumell shped rs for tensile testing were crried out t the X27C em line t the Ntionl Synchrotron Light Source (NSLS), Brookhven Ntionl Lortory (BNL). The wvelength of synchrotron rdition ws Å. A three-pinhole collimtion system ws used to reduce em size to.6 mm in dimeter. Two-dimensionl (2D) WAXD nd SAXS ptterns were collected using MAR CCD X-ry detector (MAR-USA), which hd resolution of pixels (pixel size ¼ mm). The typicl imge cquisition time ws 3 s for ech dt frme. Smple-to-detector distnce ws mm for SAXS (clirted y silver ehente, AgBe, stndrd) nd mm for WAXD (clirted y n luminum oxide, Al 2 O 3, stndrd). All X-ry imges were corrected for ckground scttering, ir scttering nd em fluctutions. The otined 2D SAXS nd WAXD ptterns were nlyzed using the POLAR softwre to otin one-dimensionl SAXS nd WAXD profiles. One-dimensionl WAXD profiles were then processed y the liner squre method to otin crystllinity. 3. Results 3.1. PPDL synthesis Previous work y our lortory demonstrted tht polymeriztion of PDL for 2 h t 7 C in dry toluene (PDL to toluene 1:2 wt/vol) with 1 w/w-% monomer-to-ctlyst gve PPDL (without frctiontion) with M n 79 g/mol (GPC reltive to polystyrene) [17]. Preprtion of PPDL with higher moleculr weights requires incresing diffusion constrints tht slow chin propgtion nd decresing rection wter content to strt fewer chins [21,24 26]. To prepre series of PPDLsmples(1 3,Tle 1) with reltively low moleculr weight, the rection temperture ws mintined t 7 C. Furthermore pre-drying of the rection flsk, enzyme nd solvent ws not crried out, while mixing ws performed y mgnetic stirring. After 1 h of rection, the viscosity of the rection mixture ws sufficiently high which stopped mgnetic stirring. Hence, the diffusion monomer in the rection system ecme slow, which hindered the increse of moleculr weight [17]. The moleculr weight of the product within the series of smples 1 3 ws only vried y the rection time. By incresing the rection time from 6 to 13 nd 24 h, of PPDL fter precipittion incresed from 2. to 4. nd g/mol (determined y light scttering, see Experimentl Section), respectively. Higher moleculr weight PPDL smples 4, nd 6 were prepred y incresing the rection temperture to 8 C nd using n overhed stirrer, oth with the intent of incresing diffusivity of rectnts. In ddition, the rection flsk, syringes for rectnt trnsfer, PDL, toluene nd Novozym 43 were ll dried prior to rections (see Experimentl Section). By incresing the rection time from 8 to 16 nd 26, smples 4, nd 6 were prepred hving vlues (1 4, determined y light scttering) of 18.8, 28. nd 48.1 g/mol, respectively. For the ove, PPDL ws synthesized in quntities up to 4 g nd yields from 7 to 8% fter precipittion Tensile testing Dumell shped smple rs were prepred (see Experimentl Section, ove) for tensile testing. Smple 1 ( )ws found to e too rittle to e tested. Stress strin curves for Smples 2 6 re illustrted in Fig. 1( e). Fig. 1f displys four regions ccording to slope chnge in the stress strin curve. Mnson et l. [27] ssigned these four regions s: I liner nd nonliner viscoelsticity, II neck region/strin softening, III plstic flow, nd IV strin-hrdening. The Young s modulus ws determined s the slope of stress strin curve t strin elow 1% using the liner lest squre method. The vlues of strin nd stress t yield were the (x, y) coordintes t which the first derivtive of the stress strin curve ws zero. Vlues of elongtion nd stress t rek represented the (x, y) coordintes fter which the slope of the stress strin curve ecme negtive. The verged results from replicted experiments re summrized in Tle 2. Inspection of PPDL smples efore stretching nd fter frcture showed tht the volume of the r sujected to high strin ws found to e out two times tht of the originl volume (i.e. % polymer nd % void formtions). This oservtion is in greement with tht y Wrd [28]. Since the stress vlue in Fig. 1 ws estimted y the lod divided y the initil cross-sectionl re, for smples 4,, nd 6, the true stress vlues t rek were reclculted using the vlue of stress t rek (in Fig. 1) multiplied y fctor (1 þ strin)/2. For smples t lower strins (smples 2, 3), the true stress vlues t rek were reclculted y the vlue of stress t rek multiplied y fctor (1 þ strin), i.e., neglecting the effect of volume increse t lower strins. The reclculted vlues of true stress t rek re listed in Tle 2. Vritions of Young s modulus, true stress t rek, elongtion t rek, nd strin nd stress t yield s function of PPDL moleculr weight re shown in Fig. 2( e). The properties shown in Figs. 1 nd 2 nd Tle 2 revel unmiguously tht they re dependent on PPDL moleculr weight. Fig. 1 illustrtes tht, for smple 2 ( ¼ ), stress first incresed rpidly with strin ut then the slope of stress strin curve egn to decrese. When strin reched 4..8%, the smple frctured (the frcture ws homogeneous s is typicl for rittle frcture), while no neck ws oserved efore frcture. As incresed to , the shpe of the stress strin curve (Fig. 1) devited sustntilly from tht of smple 2. For smple 3, necking nd plstic flow were oserved during stretching; stress first reched the mximum vlue nd then decresed, followed y stle propgtion efore frcture. When the smple r length reched 237 2% of the initil length, the smple r frctured. Thus, with n increse in the moleculr weight of out 2 times from smple 2 to smple 3, the elongtion t rek increses lmost 3 times nd the frcture fshion of PPDL chnges from rittle-to-ductile. In ddition, stress-whitening phenomenon ws oserved during stretching of smple 3, which is quite different from smple 2. Interestingly, for PPDL of ¼ (smple 4), the elongtion t rek incresed to 6 3% while the strin-hrdening phenomenon ws lso oserved (Fig. 1c). With further increses in, smples exhiited similr stress strin curve shpe s tht of smple 4 nd vlues of elongtion t rek remined lmost constnt (e.g. 7 7% for ¼ nd 8 3% for ¼ ). This suggests tht for PPDL, it is not necessry to synthesize polymers with moleculr weights ove ¼ to ttin higher drw

5 192 J. Ci et l. / Polymer 1 (21) Stress 1 1 Stress Strin,% Strin,% c 16 d 18 Stress Stress Strin,% Strin,% e 18 f I II III IV Stress 12 6 Stress Strin,% Strin Fig. 1. Stress strin curves for PPDLs with different moleculr weights (,2;,3; c,4; d,; e,6). Scheme f shows regions within the stress strin curve of PPDL smples undergoing stretching. For ech smple, repeted stress strin mesurements on fresh PPDL smples re plotted. rtios since the elongtion t rek remins lmost constnt ove ¼ The plot of true stress t rek (tensile strength) versus (Fig. 2) showed similr trend s tht of elongtion t rek versus. The stress t rek first incresed from 21.3 MP to 6.8 MP s incresed from to nd then remined lmost constnt t high. The curve of Young s modulus versus (Fig. 2c) exhiited minimum t ¼ Young s modulus first decresed from 69 MP to 29 MP with incresing, nd then incresed to 39 MP. However, the corresponding curve of strin t yield versus first incresed with from 12. to 2% nd then decresed slightly to 17.4%. The trend of stress t yield versus is lmost opposite to tht of strin t yield versus, since the stress first decresed from 24.1 to 13.3 MP, nd then incresed to 18.2 MP with incresing Therml nlysis Therml nlysis of severl smples with different moleculr weight ws crried out fter processing smples into dumell shped rs prior to tensile testing. Fig. 3displys DSC curves from first heting (Fig. 3), cooling (Fig. 3) nd second heting (Fig. 3c) scns of PPDL smples (1 6). The trend of pek melting temperture with in heting is similr to tht in cooling curves. Vlues of melting enthlpy (ME) nd melting temperture (MT) during first nd second heting scns, nd vlues of crystlliztion enthlpy (CE) nd crystlliztion pek temperture (CPT) during cooling scns re summrized in Tle 3. Since the trends of chnge in MT nd ME during the first heting scn re similr to those during the second heting scn, only vlues of ME nd MT in first heting nd vlues of CE nd CPT in cooling re plotted in Fig. 4,.

6 J. Ci et l. / Polymer 1 (21) Tle 2 Young s modulus, correltion coefficient in otining young s modulus, elongtion to rek, stress t rek, stress t yielding, nd strin t yielding for PPDL smples of differing moleculr weight. Smple No. (1 4 ) Young s Modulus (MP) Correltion coefficient Elongtion t rek (%) Stress t rek (MP) Stress t yield (MP) ( ) Strin t yield (%) (42. 3.) (6.8 2.) (. 4.) ( ) The true stress t rek clculted fter cross-section re correction. Fig. 4 indictes tht ME first incresed from 134. J/g to J/g nd then decresed fterwrd with incresing. When ws , ME reched minimum vlue (11.6 J/g); when ws , ME climed ck up to 11.3 J/g. Similrly, MT first incresed from 97 C to 13.2 C, nd then decresed to 91.7 C, nd gin incresed to 99.4 C with increse of. Upon crystlliztion of PPDL during cooling, the chnges of CE nd CPT with were similr to those of ME nd MT (Fig. 4). Generlly, the vlue of CE ws smller thn tht of ME in the sme smple, possily due to recrystlliztion upon heting [29] X-ry mesurements To compre DSC results, wide-ngle X-ry diffrction (WAXD) experiments were performed on the sme dumell shped smple rs prior to tensile testing. Frctions of crystlline phses plotted in Fig. were otined y pek deconvolution of integrted WAXD profiles. The trend of crystllinity chnge with ws similr to the vrition of ME nd CE with. To compre the DSC nd X-ry results, the vlues of crystlliztion enthlpy from cooling (insted of the melting enthlpy) were used in order to eliminte the effect of recrystlliztion during DSC scnning. The equilirium melting enthlpy of PPDL hd een estimted to e 264 J/g nd 233 J/g, respectively, y Wunderlich [29] nd Leedv et l. [3]. If 233 J/g ws used to clculte crystllinity vlues from DSC results, clculted crystllinity vlues would e higher thn those from WAXD for smples with less thn while the results were reverse when ws ove If 264 J/g ws used, the clculted crystllinity from DSC ws slightly higher thn tht from WAXD for smples with less thn ; however, the clculted crystllinity from DSC ws sustntilly lower thn tht from WAXD when ws ove Considering tht the crystllinity clcultion from WAXD includes contriutions of oth crystlline nd interphse regions, while crystlliztion enthlpy from DSC is only from the crystl formtion, the crystllinity vlue estimted from WAXD should e higher thn those from DSC if the ltter does not involve recrystlliztion. Thus, the vlue of 264 J/g ws used in this study for clcultions of PPDL crystllinity from DSC. 3.. Dynmic mechnicl nlysis (DMA) DMA ws used in previous work [18] to mesure the glss trnsition ( 27 C) nd the effect of wter content on nd g relxtions for PPDL with M n ¼ In this study, DMA ws used to compre the storge modulus of PPDL with different vlues in the temperture rnge from 3 Cto9 C. Since smple 1 ws rittle, only smples 2 6 were sujected to DMA tests. Fig. 6 illustrtes curves of storge modulus vs. temperture for PPDL smples with different moleculr weights. For ech smple, the storge modulus decresed with incresing temperture nd no distinct trnsitions were oserved. The results in smples 3 nd 4 exhiited n intersection t 8 C, while smples nd 6 exhiited n intersection t 6 C. Storge moduli t 4 9 C for five smples re listed in Tle 4 nd displyed in Fig. 6. This figure indictes tht the trend of storge modulus chnges with t 4 C ws similr to tht of the Young s modulus with (Fig. 2c). Interestingly, s the temperture incresed from 4 to 6 C, the discrepncy etween smples nd 6 decresed. For tempertures ove 6 C, the storge moduli of smples nd 6 re the sme. This implies tht ove 6 C, the moleculr networks formed for smples nd 6 hve the sme elstic rigidity. 4. Discussion This section considers the effects of on PPDL mechnicl, therml nd crystlliztion properties nd compres these to those of liner PE Effects of moleculr weight on PPDL properties The effect of moleculr weight of liner HDPE on its tensile properties ws studied y Wrd et l. [22]. They reported tht Young s modulus decreses with incresing. Since PPDL hs similr chemicl structure s liner PE, its dependence of Young s modulus should show similr trend. However, The results herein pper to e different from those reported y Wrd et l. [22] For exmple, in Fig. 2c, Young s modulus first decreses to minimum (t smple, ) nd then increses. Without smple, ehvior of PPDL would e similr to liner PE. Perhps this cn e rtionlized s follows. Severl other studies hve indicted tht the degree of crystllinity is the primry fctor ffecting the Young s modulus of semi-crystlline polymers [31 34]. It ws known tht low frctions cn ct s diluents nd retrd the crystlliztion of higher frctions [29]. Furthermore, low frctions usully contriute less to the Young s modulus due to the lower possiility to form entnglements or tie molecules to trnsmit the stress [34]. In smple, the GPC spectrum clerly exhiits severl peks corresponding to low frctions (Fig. 7). These low frctions would e expected to decrese crystllinity nd lower the Young s modulus. To verify this hypothesis, the following experiment ws performed. Smple ws first immersed

7 194 J. Ci et l. / Polymer 1 (21) Elongtion t Brek,% Stress t Brek (MP) 4 2 1x1 2x1 3x1 4x1 x1 1x1 2x1 3x1 4x1 x1 c 8 d 2 7 Y oung's Modulus (Mp ) Strin t Yield (MP) 2 1 e 2 1x1 2x1 3x1 4x1 x x1 2x1 3x1 4x1 x1 Stress t Yield (MP) x1 2x1 3x1 4x1 x1 Fig. 2. Vritions of s function of PPDL moleculr weight for tensile tests t 2 C of: () elongtion t rek, () true stress t rek, (c) young s modulus, (d) strin t yielding nd (e) stress t yielding. The solid nd dotted lines in c indicte how trends in Young s modulus s function of moleculr weight chnges with nd without smple, respectively. in chloroform for 24 h t 2 C to soluilize lrge prt of low frctions. The remining insolule mteril fter thorough removl of solvent ws sujected to GPC nd DSC nlyses. The GPC chromtogrm displyed in Fig. 7 indictes tht sustntil prt of low moleculr weight frctions were removed y soluiliztion in chloroform. DSC results indicte tht, fter solvent extrction, the melting pek of smple shifted to higher temperture during oth first nd second heting scns (Fig. 8). The melting temperture, melting enthlpy for the first nd second heting, the cooling crystlliztion pek, nd cooling crystlliztion enthlpy re listed in Tle 3 for comprison with dt for non-frctionted smple. Indeed, the vlue of melting enthlpy increses fter extrction nd flls etween those of smples 4 nd 6. Thus, it ppers tht the crystllinity nd Young s modulus of PPDL hve similr moleculr weight dependence s liner PE sed on studies y Wrd et l. [28], i.e., oth decrese with incresing Young s modulus s function of crystllinity Since crystllinity plys mjor role to influence Young s modulus, their reltionship ws investigted. The crystllinity ws clculted y dividing the cooling crystlliztion enthlpy with the equilirium enthlpy vlue of 264 J/g [29]. The reltionships etween crystllinity nd Young s modulus re shown in Fig. 9. The dt set cn e represented y stright line without the extrpoltion to the origin. This ehvior ws lso oserved y Mndelkern et l. [3], who mesured the crystllinity of frctionted liner PE y density mesurement. Flory nd Yoon [3] pointed out tht the initil chin topology in the molten polymer cn e conserved during crystlliztion. In other words, the topologicl fetures such s entnglements, knots, loops nd relted structures cn e regulted in the inter-lmellr morphous region. Without question, these types of structures will ffect the elstic

8 J. Ci et l. / Polymer 1 (21) Hetflow Endo Down Tle 3 DSC results of melting point (T m ), crystlliztion pek temperture (T cp ), melting enthlpy (DH m ), nd cooling crystlliztion enthlpy (DH c ) for PPDL of differing vlues. 1 4 First heting Second heting Cooling DH m, J/g T m, C DH m, J/g T m, C DH c, J/g T cp, C (118.6) 91.7 (13.2) 99.3 (11.8) 91.4 (98.3) 92.2 (13.8) 68. (7.) Smples extrcted with chloroform for 24 h t 2 C. Hetflow Endow Down c Hetflow Endow Down Temperture Temperture Yield stress s functions of crystllinity nd crystllite thickness The plot of yield stress versus crystllinity is shown in Fig. 9, where the ehvior cn e rtionlized y the occurrence of one or oth of the following phenomen [36 38]. One possiility is prtil melting-recrystlliztion process is induced y deformtion [36]. An lterntive explntion is sed on the screw disloction theory [37,38]. The mechnism of deformtion-induced H, J/g x1 2x1 3x1 4x1 x T m Temperture Fig. 3. DSC thermogrms recorded for PPDL smples 1 6 during: () first heting, () cooling nd (c) second heting. H, J/g T cp properties. In ddition, the interphse region tht connects the ordered crystlline region nd the isotropic conformtionl disordered morphous region cn lso contriute to elstic modulus [34]. A schemtic digrm indicting different chin topologies in the lmellr structure of PPDLs with different is shown in Fig. 1, which will e used to explin the different oserved properties in the smples. 9 1x1 2x1 3x1 4x1 x1 Fig. 4. Plots of melting enthlpy, melting point (T m ), crystlliztion enthlpy nd crystlliztion pek temperture (T cp ) s function of PPDL moleculr weight recorded y DSC during: () first heting nd () cooling. 6

9 196 J. Ci et l. / Polymer 1 (21) Tle 4 Storge modulus of PPDL smples of differing t vrying tempertures. W c,% 1 4 Modulus, P 4 C C 6 C 7 C 8 C 9 C x1 2x1 3x1 4x1 x1 Fig.. Crystllinity of PPDL, determined y wide-ngle X-ry diffrction (WAXD), s function of PPDL moleculr weight. Storge Modulus 6.x1 8 3.x Temperture melting-recrystlliztion is sed on previous experimentl results y oservtion of trnsmission electron microscopy [39] nd smllngle neutron scttering [4] results with liner PE. The screw disloction theory rgues tht the sher yield stress (i.e., experimentl yield stress/2) is relted to the crystllite thickness y the following expression: s y ¼ k " exp 4p!# 1 2pDG c k 2 þ 1 (1) L c where s y is the sher yield stress, L c is the crystl lmellr thickness, k is function of the crystl sher modulus, is the Burgers vector hving the sme vlue s the PPDL unit cell c-xis, DG c is the criticl ctivtion energy for disloction growth, with vlues in the rnge etween 4 nd 8 kt. To correlte the reltionship etween yield stress nd crystllite thickness, SAXS experiments were crried out where the results (the Lorentz-corrected SAXS profiles) re shown in Fig. 11. These profiles exhiit two peks t very low q vlues. To otin the long period L nd crystllite thickness L c, the onedimensionl correltion function K(r) is clculted y the following eqution [41]: KðrÞ ¼ 1 Zp 2p 2 Iq 2 cos ðqrþdq (2) where I(q) is the scttering intensity nd r is the distnce for which the electron density correltion is mesured. The long periods for smples 3, 4, nd 6 were 17. nm, 18.9 nm, 19.3 nm nd 18.1 nm, respectively. Assuming the crystl thickness should e thicker thn the morphous regions in the two-phse model, the estimted Storge Modulus, P 8.x1 8 6.x1 8 4.x1 8 2.x1 8 4 C C 6 C 7 C 8 C 9 C Concentrtion signl(mv) Originl After extrction x1 2x1 3x1 4x1 x1 Fig. 6. () Plots of storge modulus s function of temperture for PPDL with different moleculr weight (2, ¼ 4,; 3, ¼ 81,; 4, ¼ 189,;, ¼ 28,; 6, ¼ 481,); () dependence of storge modulus on PPDL moleculr weight t selected tempertures Elution time(minutes) Fig. 7. Comprison of GPC curves for PPDL smple efore nd fter solvent extrction.

10 J. Ci et l. / Polymer 1 (21) After extrction 8 Hetflow Endow Down Originl Young's Modulus Temperture Originl X c 3 2 Hetflow Endow Down After extrction Stress t Yield Temperture Fig. 8. Effect of smple frctiontion melting of PPDL smple determined y DSC thermogrms during: () first heting nd () second heting scns X c Fig. 9. Plots of Young s modulus () nd stress t yield () s function of PPDL crystllinity. crystlline thickness vlues re 1.7 nm, 12.2 nm, 13.4 nm, 11.1 nm, respectively. The close vlues of the crystlline thickness led us to conclude tht the screw disloction theory is not pproprite to explin yield stress dt. tight-fold 4.4. Strin nd true stress t rek Severl studies [22,3,42 4] hve een reported deling with the moleculr weight effect on drw rtio for PE. The results indicte tht the drw rtio decreses (e.g. 18 3) with incresing moleculr weight of liner PE, wheres the true stress t rek vlue exhiits mximum t ¼ [33]. Results for PPDL smples re different from tht otined with PE. For PPDL, when increses to , the drw rtio reches n symptotic vlue of out 7. (Fig. 2). Interestingly, the corresponding true stress t rek lso reches n symptotic vlue of out 6. MP t ¼ (Fig. 2). One possile explntion for the discrepncy is s follows. Since the PPDL chin consists of C C nd C(]O) O (or O C ) onds long the ckone, where the C C ond hs lrger ond energy thn C(]O) O (or O C ) onds. Hence, the stress distriution should e heterogeneous long the chin during stretching. In high smples (> ), tie-chin c entnglement i i c crystlline interfcil morphous interfcil crystlline Fig. 1. Possile mechnism responsile for the rittle-to-ductile trnsition in: () lower moleculr weight frction nd () higher moleculr weight frction.

11 198 J. Ci et l. / Polymer 1 (21) Conclusions q 2 I(q) (.u.) # 6# 3# incresed would led to enhnced strength of the entnglement network in the molten melt s well s in the inter-lmellr morphous region when solidified. However, s the C O ond is weker thn the C C ond, the former proly reks t lower stress under deformtion, thus limiting the ultimte strength of PPDL. This hypothesis is consistent with the oservtion of constnt stress s well s the constnt elongtion t rek vlue ove ¼ under deformtion. This my lso explin why PE hs much higher drw rtio (18 or higher) thn PPDL of the sme or higher, s the PE chin hs only cron cron onds [32].In generl, vritions of elongtion nd true stress t rek (tensile strength) with oserved in this study re consistent with those reported in tensile study of cellulose cette y Sookne et l. [4] nd the theoreticl prediction of Flory [46]. 4.. Brittle-to-ductile trnsition.1.2 q(nm -1 ) Fig. 11. Lorentz-corrected SAXS profiles of PPDL smples 3 6 efore stretching. The rittle-to-ductile trnsition in semi-crystlline polymers cn e ffected y mny fctors such s deformtion rte nd crystllinity [28,34]. Usully, increses in deformtion rte nd crystllinity will cuse the rittle-to-ductile trnsition to shift towrds higher. However, moleculr weight is still the most importnt fctor tht ffects the mteril if the rittle-to-ductile trnsition is oserved. Mndelkern et l. [3] found tht for ,PE mterils re rittle over the ccessile rnge of crystllinities. When is , the rittle-to-ductile trnsition cn e oserved. Furthermore, when the crystllinity is higher, the trnsition ecomes shrper. In higher PE (out ) the rittle-to-ductile trnsition ws not oserved y incresing PE crystllinity. For PPDL, the t which the rittle-to-ductile trnsition occurs (Fig. 2) is in greement with the rnge oserved for PE. In ddition, quenched PPDL smple 3 exhiited ductile ehvior, while the nneled smple with higher crystllinity reveled rittle ehvior. This finding grees with Mndelkern s experiments on liner frctioned PE with ¼ [3]. Finlly, we cution tht when ssessing the mechnicl properties of PPDL smples of different moleculr weights, in ddition to crystllinity, crystllite thickness, inter-lmellr thickness nd density of physicl entnglements in the morphous region, the frction of the interfcil region nd supermoleculr morphology should lso ply n importnt role [34,47,48], which will e considered in our future studies. 4# Synthesis of lrge-scle PPDL smples ws performed y lipse ctlysis. Vrition of PPDL from 2. to ws chieved y mnipulting rection vriles including rection wter content, method of mixing nd rection time. Tensile testing, DSC, X-ry, DMA nd GPC were used to investigte the effect of moleculr weight on mechnicl, therml nd crystlline mteril properties. Cold-drwing tensile tests t room temperture reveled rittleto-ductile trnsition for PPDL smples with vlues etween nd For PPDL with ¼ , the entnglement network strength in non-crystlline regions is not sufficiently high to trnsmit stress during stretching, wherey interfirillr slippge domintes until frcture. As PPDL is incresed from nd ove, the entnglement network strength is gretly enhnced nd strin-hrdening tkes plce t high strins prior to filure. Furthermore, the elongtion t rek nd tensile strength (i.e., true stress t rek) rech symptotic vlues of 6% nd 6.8 MP, respectively. The trends for chnges in Young s modulus, melting enthlpy (from DSC) nd crystllinity (from WAXD) s function of moleculr weight re ll similr. Anormlity of one smple () ws explined y its contmintion with high levels of lower moleculr weight frction. Storge modulus (from DMA) reveled similr moleculr weight dependence trends t tempertures elow 6 C. However, ove 6 C, storge moduli of higher moleculr weight smples ecomes indistinguishle, indicting tht the crystlline network structures of these smples t higher tempertures re lso similr. Overll comprisons etween PPDL nd liner high density polyethylene (HDPE) mechnicl properties showed similr trends in Young s modulus with moleculr weight, ut differing trends with respect to elongtion t rek nd true stress t rek s function of moleculr weight. The ltter differences re explined y the presence of low ut persistent density of C O onds in ester links of PPDL tht re not present in PE. We therefore conclude tht PPDL-like polyesters hve excellent potentil to function in similr wys to PE. This odes well to the potentil further development of similr or relted polymers for commercil use. Indeed, our lortory hs developed ioctlytic methods using n engineered Cndid tropiclis strin to convert ftty cids, such s tetrdecnoic cid, in volumetric yields of up to 16 g/l to their corresponding u-hydroxyftty cid (e.g. u-hydroxytetrdecnoic cid) (mnuscript in review). Susequent conversion of u-hydroxyftty cids to polyesters y condenstion polymeriztion will provide low-cost sclle route to iosed PPDL-like mterils tht cn function in similr wys to PE. Acknowledgements The uthors thnk the Ntionl Science Foundtion (NSF) nd industril memers of NSF-I/UCRC for Bioctlysis nd Bioprocessing of Mcromolecules t NYU/Polytechnic University for finncil support, intellectul input, nd encourgement during the course of this reserch. BH lso thnks support from NSF (DMR- 9612). We re lso grteful to Mr. Seong Chn Prk nd Dr. Mirim Rfilovich t SUNY t Stony Brook for providing ccess to their Instron Tensile testing nd DMA equipment. References [1] Mzzocchetti L, Scndol M, Jing ZZ. Mcromolecules 29;42:7811. [2] Ceccorulli G, Scndol M, Kumr A, Klr B, Gross RA. Biomcromolecules 2;6(2):92. [3] Jing ZZ, Azim HS, Gross RA, Focrete ML, Scndol M. Biomcromolecules 27;8:2262. [4] Keller A, Mrtuscelli E, Priest DJ, Udgw Y. J Polym Sci Prt A ;9:187.

12 J. Ci et l. / Polymer 1 (21) [] Ptel GN, Keller A. J Polym Sci Polym Phys Ed 197;13(2):33. [6] Hridoss S, Perlmn MM. J Appl Phys 1984;:1332. [7] Sezgin A, Ch WS, Smith JM, Mccoy J. Ind Eng Chem Res 1998;37(7):282. [8] Zhng JG, Jing DD, Wilki CA. Polym Degrd St 26;91(4):641. [9] Fng CQ, Li TH, Zhng ZP, Jing DQ. Polym Comp 28;29():. [1] < [11] Meulen I, Geus M, Antheunis H, Deumens R, Joosten EAJ, Koning CE, et l. Biomcromolecules 28;9:344. [12] Jedlinski Z, Juzw M, Admus G, Kowczuk M. Mcromol Chem Phys 1996;197:2923. [13] Mkovetsky KL. Vysokomol Soedin Ser B 1999;41(9):12. [14] Yevstropov A, Leedev BV, Kiprisov Y. Vysokomol Soedin Ser A 1983;2: [1] Zhong Z, Dijkstr PJ, Feijen J. Mcromol Chem Phys 2;21:1329. [16] Uym H, Tkey K, Koyshi S. Bull Chem Soc Jpn 199;68:6. [17] Kumr A, Klr B, Dekhtermn A, Gross RA. Mcromolecules 2;33:633. [18] Focrete ML, Scndol M, Kumr A, Gross RA. J Polym Sci Prt B Polym Phys 21;39:1721. [19] Simpson N, Tkw M, Hult K, Johnsson M, Mrtinelle M, Mlmström E. Mcromolecules 28;41:3613. [2] Gzzno M, Mlt VA, Focrete ML, Scndol M, Gross RA. J Polym Sci Prt B Polym Phys 23;vol. 41:19. [21] Focrete ML, Gzzno M, Scndol M, Kumr A, Gross RA. Mcromolecules 22;3:866. [22] Cpccio G, Wrd IM. Polymer 197;16:239. [23] Mtthijs de Geus. Enzymtic ctlysis in the synthesis of new polymer rchitectures nd mterils. Ph.D. thesis. Eindhoven University of Technology, Netherlnds; Fe. 27 [chpter ]. [24] Kumr A, Gross RA. Biomcromolecules 2;1:133. [2] Bisht KS, Henderson LA, Gross RA, Dl Kpln, Swift G. Mcromolecules 1997;3(9):27. [26] Mei Y, Kumr A, Gross RA. Mcromolecules 22;3:444. [27] Mnson JA, Hertzerg RW. CRC Crit Rev Mcromol Sci 1973;1:433. [28] Wrd IM. Mechnicl properties of solid polymers, vol John Wiley & Sons Pu; 1983 [chpter 12]. [29] Wunderlich B. Advnced therml nlysis. Dtnk of thermodynmic properties of liner mcromolecules nd smll molecules. Knoxville, TN, USA: Deprtment of Chemistry, University of Tennessee; 199. [3] Leedev B, Yevstropov A. Mkromol Chem 1984;18:123. [31] Keith HD, Pden FJ, Vdimsky RG. J Appl Phys 1971;42:48. [32] Cpccio G, Crompton TA, Wrd IM. J Polym Sci Prt B Polym Phys 1976;14:1641. [33] Mndelkern L. Polym J 198;17:337. [34] Mndelkern L. Acc Chem Res 199;23:38. [3] Kennedy MA, Pecock AJ, Mndelkern L. Mcromolecules 1994;27:297. [36] Flory PJ, Yoon DY. Nture 1978;272:226. [37] Young RJ. Philos Mg 1974;3:8. [38] Crist B, Fischer CJ, Howrd PR. Mcromolecules 1989;22:179. [39] Philips PJ, Philipot RJ. Polym Commun 1986;27:37. [4] Wu W, Wignll GD, Mndelkern L. Polymer 1992;33:4137. [41] Strol GR, Schneider M. J Polym Sci Prt B Polym Phys 198;18:1343. [42] Andrews JM, Wrd I. J Mter Sci 197;:411. [43] Willimson GR, Wright B, Hwrd RW. J Appl Chem 1964;14:131. [44] Popli R, Mndlkern L. J Polym Sci Polym Phys Ed 1987;2:441. [4] Sookne AM, Hrris M. Ind Eng Chem 194;37():478. [46] Flory PJ. J Am Chem Soc 194;67:248. [47] Peterlin A. J Mter Sci 1971;6:49. [48] Vincent PI. Polymer 196;1:42.